Video Science - Experiments

Adding heat energy to gas particles causes them to move faster, spread apart and occupy a larger amount of space (in science, called volume). Learn about how this applies to hot air balloons and why they float.

Adding heat energy to gas particles causes them to move faster, spread apart and occupy a larger amount of space (in science, called volume). In calculating the density of matter through the formula
Density = Mass/Volume
an increase in volume, increasing the volume of gas by heating the sample and accelerating the particles causes a decrease in the density of the sample. The heated, less dense gas is then displaced by the cooler, denser gases surrounding the sample (here contained in a thin metallic polyester sleeping bag).

“Containing” the sample by tying off or sealing the gas in the survival bag creates a sealed mass of warm gas that can be “launched” around the laboratory to move around on top of a layer of cooler ambient or surrounding gases. Heat energy, moving from hot to cold, will slowly leave the sample until its temperature equals the temperature of gases surrounding it. As the gas in the silver bag cools, the bag slowly drops to the floor.

More Science Please: Observant students will comment (correctly) that you are not simply heating the gas, but also mechanically pumping it into the sleeping bag. Ask the class to propose a test to see if the heat is in fact the factor affecting the density of the gas. One simple method of testing the effect may be to fill an identical bag with cool gas by adjusting the fan setting on the hair dryer to high, yet the temperature setting to low. The two silver bags, inflated side by side, should exhibit different densities according to the temperatures of the gases inside them. You may also simply repeat the experiment with the hairdryer set at a cool setting, yet allow the bag to return to room temperature before doing so. You may also test the cool gas, your control setup, first to omit the hidden variable of heat energy remaining in the bag (an insulator) after the heated trial.

If you are running this experiment, try to cool the classroom first to amplify the temperature gradient between the contained and ambient gases. Alternatively, you may take the class to an unheated space if possible, or outside (tie the bag with kite string to avoid losing it in the wind and creating a possible hazard if the sleeping bag drifts into traffic).

But what if…: If you can’t find a silver polyester emergency survival sleeping bag, substitute a large construction debris bag. If you are running this experiment on a cool but sunny day, the black plastic sack may launch even if it is inflated with cool gas because the infrared radiation of the sun will be absorbed by the dark surface of the bag. If you can find a silver polyester safety blanket, you may seal if on three sides to fabricate an inflatable structure, yet use a thin stream of adhesive to avoid adding too much additional mass to the structure and inadvertently making it too difficult to “launch” in the process. There are similar products marketed by science supply companies including a very long “solar” black tube, yet it’s quite expensive, fragile, and normally works only on the most calm, still and bright days.

Concept notes coming soon.

Of all the iterations we see of science lessons preparing “Silly Putty” from borax and glue, we’d like to see students looking carefully at each aspect of this classic chemical reaction, including the solution chemistry and the simple math involved preparing each reactant.

Sodium tetraborate (Borax is also termed Granubar^md), the common laundry detergent additive, lends itself well to this experiment because it is quite easy and safe to handle in the laboratory. It is a white crystalline solid with a chemical formula of Na₂B₄O₇*5H₂O (the *5H₂O indicates that the compound is in the pentahydrate form, as were our copper sulfate crystals in our Zinc/Copper redox segment). It is sometimes used in farming to reach the correct boron concentration in soil, yet more commonly used as a laundry detergent supplement. We use a 4% solution in our segment that is “scaled up” to produce a 2 liter sample, yet in a classroom situation, you may elect to have students add 4 grams of the solute to 96 ml of distilled water to the same effect. You may also lower the concentration of borax to 3% (3 grams solute to 97 ml solvent) if you find (as we did) that the solution saturates at 4%.

Alternatively, you may elect to warm the distilled water to drive the full 4 gram sample of solute into solution, or perhaps introduce a variable into the experiment by asking six teams to prepare samples at increasing concentrations (Team 1 – 1%, Team 2- 2%, etc.) and then gather the class to evaluate the six product samples to judge which concentration was optimal based on the stability of the final product. In dissolving the glue by 80%, we model adding the glue to the distilled water in a wide mouth Nalgene^tm graduate or pitcher. In preparing a classroom (student) version of the experiment, we would scale the sample down to 50 ml of glue diluted with 40 ml of distilled water. As the glue is nontoxic and water soluble, we find it manageable to work with in a science lab, even with smaller children. In lieu of a glass or Pyrex stirring rod, we can recommend a wood splint or popsicle stick if teachers are concerned with breakage as students stir the reacted polymers (quite thick and viscous, potentially causing the glass rod to “snap” if overeager students stir the reaction vigorously. Student will note the good solubility of the glue in the distilled water, and you may take a moment to comment on the pigmentation of the glue (titanium dioxide is in fact the same material that is added to skim milk to make it appear “whiter”, richer and more appetizing).

Rinse in the Water Clear…: Rinsing the reacted product in tap or distilled water will wash away any unreacted sodium tetraborate. Without rinsing, you may find your sample is tacky and leaves the skin of your hands dry by sapponifying the fats and oils in your skin (aka “dishpan hands). Yet the rinsed sample, with some stretching and kneading, can be formed into a clean, pliable sample of a white amorphous solid that kids will enjoy handling because of its “nonNewtonian” properties (it behaves like a liquid under some conditions, as a solid under others). Students normally ask to keep the reacted samples. If you do send them home, send them sealed in a plastic bag and keep them in your lab until the end of the day to avoid student temptation to handle the material in other classes. Caution the students to avoid letting the reacted polymer come into contact with carpet, upholstered surfaces or fabrics (it can stick to fibers). If a sample becomes dry and brittle, it can be rehydrated with a few drops of distilled water. You may choose to tint the solution or the dissolved glue with vegetable dye, yet we normally do not to avoid staining our hands when handling it.

Bonus! Adding your extra sodium tetraborate solution to your dissolved polyvinyl alcohol bags (see our segment titled “Slime” will cross link the polymers in that experiment, producing an amorphous yet clear final product that can be trained to climb down a windowpane through the effects of gravity and surface tension. Stirring your solution with a highlighter will allow your final sample to fluoresce under a black light.

Concept notes coming soon.

Our version of a manipulative model developed by NASA in the early 1990’s to illustrate the mechanism by which engineers protect astronauts from the extreme heat of the sun during spacewalks and remote operations is designed to get students thinking about aerospace science.

For younger students, two whole class demonstrations (one with and one without the internal cooling mechanism) can generate data for a simple double line graph tracking the temperature of the cans as they are exposed to the heat lamp (a “green version” of the experiment can be done outside on a sunny day with a garden hose or simple siphon to cool the model in lieu of water pumped from the sink and heat generated by the infrared bulb).

Students normally run two consecutive data trials of 10 minutes each, which allows time within a 40-50 minute science lesson for the setting up and dismantling of the equipment. When the temperature of the setup remains unchanged for three consecutive readings, students may elect to stop testing. The indoor/science lab version of the experiment tends to generate a more uniform heating curve and data of slightly higher integrity. Ask your students why (the infrared rays of the sun a normally filtered by clouds and particulates in the atmosphere).

Some Variations:

1. You may try using a thinner radius of tubing and smaller can (soup can in lieu of the coffee can). This may enable you to run six trials of the experiment with six teams in class and compare their findings en masse. Also, the internally cooled smaller can with a thinner cooling coil would have less mass, requiring less magnetic force to attach it to the ring stand (you may choose to simply stand the can on a smooth surface if you are unable to find a rare earth magnet strong enough to suspend it on the ring stand. You may also use a large pipe clamp from the hardware store in lieu of the magnet).

2. Ask students to evaluate the data and to propose improvements to the model. Advanced learners may comment on the inefficiency of using a rubber tube to carry the water through the through the center of the temperature controlled can (rubber is an insulator, which limits the effectiveness of the cooling mechanism). Students may propose substituting copper tubing (expensive and more difficult to work with yet a better conductor of heat energy).

3. Try to capture and recycle the effluent, if even to water classroom plants with it. I use a 5 gallon construction pale and run the faucet slowly to conserve the amount of water used in the experiment. The water simply needs to move through the coils continuously, but it can do so slowly.

4. Keywords for further investigation: Infrared Ray, Radiation, Vacuum, Conduction, Insulator, Conductor, Siphon, Hyperthermia.

5. Extension: Purchase a toddler’s snowsuit from a thrift store and thread the thin aquarium tubing through it between the outer shell and inner lining. Test your prototype in the same manner with either a meat thermometer placed inside the suit or ideally with a Vernier temperature probe interfacing with your computer. It’s visually exciting to design and build a space suit simulator in the shape of a human form, and a toddler’s snow-suit makes an excellent scaled down prototype for testing.

6. On a very cold day, siphon or pump warmed water through the suit and attempt to reverse the process, maintaining a comfortable body temperature in an extreme cold setting to prevent hypothermia. You may also test your prototype in a chest freezer if you have access to one in your school’s dining hall.

7. Reading Further: Find biographical accounts of the first astronauts’ early space walks and learn about experiences and sensation of the conditions in outer space. If you’re lucky, ask an astronaut in person at an air & space museum or a school visit.

We want you to examine the reason certain materials feel colder than others, so we offer an experiment that involves a qualitative examination of materials chosen for their conductive properties.

We know that heat energy will flow from hot to cold in an effort to reach the most stable (lowest energy) state. Yet the rate at which heat energy leaves a warmer body (here the feet of a science student) is felt as “coldness” in metals (good conductors) more clearly than it is in “warmer” feeling materials such as carpet and wood (poorer conductors of heat energy). The highlight in this particular experiment is the specially engineered “superconductive” plate made of an alloy (or solution of metals) that allows heat to move quickly through it. Our “Thawing Plate” feels ice cold to a science student’s bare foot because it allows heat energy to move swiftly through it. We use aquarium thermometers to show students that the materials are actually the same temperature, yet feel “warmer” or “cooler” depending on the rate at which heat energy passes from your bare feet through each material in the walk of conduction.

Extension: Quantify the rate at which heat energy moves through the “Superconductive plat” by standing a push pin in a drop of melted wax or petroleum jelly. Then gently warm the opposite end of the metal plate over a tea candle and measure how long it takes for the heat of the candle to travel through the plate, melt the wax or petroleum jelly, causing the pin to fall over. Compare this time to that needed for the head energy to pass through the marble slab. For data on the combustible materials, you may want to dip one end of the sample in very hot water and measure the temperature increase with an aquarium thermometer on the far end of the sample. In porous materials, you may substitute a hair dryer in lieu of the hot water, with settings on high heat and low fan, taking care to aim the dryer at the bottom of the sample so as not to warm the thermometer directly.

Want some More?: Carefully glue thermosensitive paper to the top surface of the wood, cardboard and stone surfaces and use the color change to observe the rate at which the added heat energy moves through these materials when they are heated with the hair dryer. Hidden Variables: Run this experiment in a shaded indoor location to eliminate radiant heat energy from interfering with your results. Students may also suggest painting each material the same color (I have used green, blue, gray) to normalize the rate at which they absorb any ambient infrared radiation.

In teaching about the behavior of light, the laser is one of the most dramatic and effective teaching tools. The cost of lasers has come down significantly in recent years, to the point where colored lasers that were once extremely expensive can now be purchased quite reasonably.

Our new unit emits a brilliant, crisp beam of green amplified light. Our first use here is a simple illustration of Hero’s Law of Reflection. After firing the laser at a mirror, we use atomized water to scatter the laser beam. Our camera is able to film the laser beam as it interacts with the microdroplets of atomized water. You can see in the film that the angle at which the laser strikes the mirror in our dark lab is the same as the angle at which the reflected ray bounces back. You could measure the angles of incidence and reflection using an oversized protractor similar to those used when drawing geometric figures on a white board during a math lesson. Alternatively, you can take a digital photo of the laser beam’s reflection from the mirror (disable the flash on your camera) and then calculate the angles from the photo.

While canned stage fog can be used to “visualize” the laser, we prefer atomized water because it’s less expensive and odorless. Stage for is more effective for some experiments because it remains suspended in the air, allowing more time to interact with the laser beam. We do not recommend using chalk dust or any other airborne particulates to scatter the laser beam because of the risk of triggering asthma attacks and respiratory problems.

We plan to add other laser experiments with our new green laser unit in the near future.

Concept notes coming soon.

Concept notes coming soon.

Concept notes coming soon.

This is the first of our experiments called "The Alien Egg". In these experiments we dissolve the shell of an egg and then submerge the egg in solutions of different solutes, with different concentrations. The membrane of the egg remains intact, resulting in the egg expanding or contracting as water enters or escapes. While not really an "alien egg", it looks like one and kids enjoy this experiment a lot.

Phase one of "The Alien Egg" relies on the solubility of calcium carbonate in acetic acid.

Here we pre-treat eggs by buffing the shells lightly with superfine steel wool to remove some of the food wax that is sometimes sprayed onto eggs when they are processed for sale. We then rinse away any remaining wax residue with a product called vegetable wash, which is intended to remove food grade waxes from produce. You can quantify the time saved by preparing your Alien Egg specimens this way by running a control setup that consists of an untreated egg soaked in acetic acid under the same conditions (temperature, light) as our treated specimens. The reaction time for the treated egg is typically one third that of the untreated version.

In a warm room, the eggshell is typically ready for removal after 45 minutes. Once the calcium carbonate shell has softened and corroded in the acetic acid, you may rinse off the remaining shell under warm water by rubbing (very gently) with your finger tips. Attempting to scratch away the calcium carbonate reside normally results in tearing the delicate inner membrane we are trying to expose. If you are teaching a large class, you can “batch process” a few dozen eggs in a long, flat clear plastic storage container but setting the eggs into a bath of vinegar before leaving at the end of the day, then rinsing off the membranes on the following morning. To remove the smell of vinegar from the storage bin, fill it with warm soapy water, seal it, and shake it vigorously.

2CH₃COOH + CaCO₃ -> Ca(CH₃COO)₂ + H₂O + CO₂

You may notice the carbon dioxide bubbles forming quite quickly if your classroom is warm and if you have pre-treated the eggshells with the steel wool and vegetable wash.

Besides water and carbon dioxide, the reaction produces a compound called calcium acetate, Ca(CH₃COO)₂, a common food additive used as a stabilizer in the manufacturing of candy products like taffy and also in preserving the texture of canned vegetables.

Concept notes coming soon.

Concept notes coming soon.

Concept notes coming soon.

A classic, easily replicated demonstration of the principles involved in internal combustion of a fuel. An arc of electrons traveling through a sealed canister of fuel activates a chemical reaction, releasing heat and light energy.

The elastic seal of the canister’s "snap-on" airtight top allows enough pressure to build from the heated, expanding gases to provided a launch force that is strong enough to propel the canister a distance of a few meters. In a darkened classroom, students may observe a flaming "tail".

Safety Parameters:
1. Wrap the igniter/cable connection carefully in electrical tape to avoid a painful electric shock.
2. Use only a tiny amount of fuel. I recommend using a simple, disposable dropper to add one or two drops in lieu of dipping your finger in the fuel and rubbing it along the edges of the container. If there is unreacted fuel still burning in the canister when you retrieve it after launch, the remaining flame may ignite the fuel residue on your fingertip.
3. Aim canister away from glass structures and combustibles and away from the classroom door.
4. Store fuel or perfume in a flammable storage unit preferably in a separate room.
5. Lock your igniter away after the lesson and avoid leaving the igniter unattended. Unwitting students could accidently shock themselves or another student with it.
6. Watch carefully where the canister lands after being launched and avoid attempting to pick the canister up with your bare hands. It may continue burning for several seconds after landing and could ignite nearby combustibles. So quickly retrieve it wearing a heat resistant glove similar to the ones shown in our video.

Part of a series of polymer chemistry lessons and experiments under development for Video Science, we offer this particular segment as a "tickler" to encourage students to consider the chemistry and materials science involved in the objects and tools they handle day to day.

The counterintuitive effect of the melting "plastic" is intended to get students thinking about the potential for green engineering of other items. What if all plastics could dissolve and wash away when their intended uses have been served? Are there similar materials in use? Can your students engineer their own green approach to storing, sealing and handling materials at home and school?

Regarding the simple chemistry presented here, the blue "slime" handled here is a polymer, or large molecule built of identical components that are linked together in a chain. A second phase of this activity will entail "linking" the chains to prevent them from sliding over each other (as seen in our footage), "stiffening" the matter into a more viscous form commonly known as "slime". Adding a 4% solution of sodium borate (made by stirring 4 grams of laundry booster into 96 ml of warm water) will allow the borate ions to "lock" the polyvinyl monomers (chemical chain "links") into position in a chemical reaction called a cross linkage or cross link polymerization.

Retain samples of each reactant and ask students to compare the properties of the reactants and product in a simple matrix to show, in a structured way, the qualities of each component in this investigation.

Heating our sugar cubes to 186 degrees Celsius with our butane torch leads to a chemical reaction called pyrolysis and the formation of caramel, carbon dioxide and water, which we see as a pale brown steaming syrup with a familiar, pleasant nutty odor.

The version shown here is an easy way to demonstrate several indicators of a chemical reaction for students who are learning to differentiate between chemical and physical (or reversible) changes. We offer the blowtorched sugar cube as a clear example of a chemical change because students can see a clear color change and sense the odor of a chemically distinct substance that is produced by the breaking and reconfiguration of chemical bonds in the sucrose.

On the other hand, a physical change may involve simply dissolving the sugar cubes into hot water to create a supersaturated sucrose solution, and observing the crystals formed as the water evaporates in the span of a few days. The crystals retain their chemical identity when simply dissolved, yet take on new properties when burned and heated above 186 degrees. The temperature at which this pyrolysis reaction occurs is a property that can be used to identify an unknown sample of sucrose in a lab with older students.

Concept notes coming soon.

Concept notes coming soon.

A very simple and inexpensive black light can be easily configured from a "Clamp Lamp" from the hardware store and an inexpensive UV light from the reptile section of a pet shop.

Your students can use the black light module to examine samples to phosphor containing compounds such as detergents, eye drops, tonic water and nontoxic fluorescent dyes from a highlighting pen soaked in distilled water. If your students stir a container of very cold distilled water with fluorescent highligher, then pour the cold liquid into a beaker or warm distilled water under the inexpensive black light unit shown in this illustration, they will see very clearly the principle behind a thermocline, or the action of warm and cold liquids and the heat energy moving through the liquids through convection currents. I call this simple demonstration a Neon Spiral and ask kids to predict what they will see beforehand in a simple sketch and explanation for their science notes.

The "Clamp Lamps" are very inexpensive, simple to assemble and use, suitably durable for lab experiments and easy to store as well. I like to keep them in all of my science classrooms and use them often when my students are studying the behavior light rays and nature of light energy.

We return to our chemical reaction one day later.

This experiment involving copper sulfate and mossy zinc is a very visual demonstration of a chemical reaction. It can be used to teach about redox chemistry in a qualitative or quantitative way. If you monitor the reaction over a period of about 24 hours, you will also see a dramatic color change and change in temperature

These bubble tubes are beautifully engineered pieces of science equipment for physics labs. You can get a set where each tube is filled with a liquid of different viscosity. Try filling them with your own liquids and have your students guess the liquid by the speed at which a bubble moves down the tube when it is angled at an incline.

This is a classic kitchen chemistry experiment involving the liberation of hydrogen gas.

We return to the do-it-yourself barometer experiment, this time using a large plastic bottle from the grocery store.

Turn your classroom into a weather station by making your own air pressure barometer from cheap and recyclable materials. Watch the water level go up and down each day as the air pressure changes.

If you're teaching light physics to middle school kids, you can show what happens to optical images when light passes through a pinhole aperture. Students love this experiment and it's very simple to setup. For homework, get your students to construct a simple ray diagram showing how the rays of light enter the pinhole viewer and form the image. You can also incorporate a lesson on how the human eye does the same.

You can teach your students about the density of different gases - carbon dioxide and air - by showing them how bubbles levitate on top of a layer of carbon dioxide. This is a gorgeous experiment and the materials are simple and cheap.

This is a simple experiment you can do for a bit of fun. Aim your "air cannon" at the flame and try to extinguish it. Add a laser on top for aiming accuracy and have a contest for fun!

This experiment is a variation on the first carbon dioxide race, where this time we watch the carbon dioxide extinguish the flames from the bottom of the tank to the top as a thicker layer builds up and spills over the beaker. Why does a layer build up from the bottom? Ask your students - they may remember from an earlier experiment

This experiment looks at how the volume of a gas changes with heating and cooling. You can make a cold bath using isopropyl alcohol and dry ice (the alcohol can get very cold - much colder than water - without freezing). Then take a water bottle and run it under hot water. Put the cap on, submerge it in the bath and see what happens.

For a fun addition to the previous experiment, add some liquid soap or detergent to generate a giant spout of bubbles. Pop the bubbles to release carbon dioxide gas, or leave the experiment to bubble away - it can last for hours.

Add dry ice to water and watch it sublime! Put some food coloring into the water for an eye-catching demonstration. You can collect the gas in a large balloon and calculate the volume of gas by measuring the circumference of the balloon.

If your students are studying the chemical properties of gases as part of a greater study of matter, elements, compounds and their chemical properties, you can easily show the ability of carbon dioxide to extinguish a flame.

As your school or library discards old videocassettes, consider using them in a simple engineering lesson before recycling the materials in them.

Assemble your students into teams and examine the video cassettes. They can predict how many individual parts are involved in its operation. Similarly, they may estimate the length of the tape. Once the prediction and estimate is complete, allow students to disassemble the cassettes, recording and sketching the components as they work through the process. As a thinking exercise, ask them to propose alternative materials that could have been used in place of the original. Perhaps they will propose greener options. As they work, see how they are able to process, manipulate, disassemble and keep track of the tiny and delicate structures.

I provide a bin for them to store the parts if they want to return to the project at a later time. Also, a film cannister provides a convient place to store the screws, spindles and smaller components.

Ask the teams to re-assemble the videocassettes, invite comments on the efficiency of the design and materials science involved with each component. If possible, ask each team to reveal the approximate date of manufacture for their cassette and the number of parts contained in it. Plot the date vs number of parts for each team's cassette and try to identify a pattern in this data. If you have several classes completing this activity, aggregate the data for all of them into one graph.

Charles' Law states that the volume of a gas is proportional to the temperature of the gas. Let your students figure out this law experimentally, by seeing what happens when a marshmallow or a bar of ivory soap (hint this soap works best as it is full of air bubbles) is put in the microwave.

Scaling up an experiment allows students to compare the data gathered from different forms of lab work to see if, how and why results may change when the size of the experimental components is altered.

Scaling up and experiment requires students to look at the logistics of an experiment and determine the best fit and application of the tools available to them. It's very good practice for students interested in engineering and also forms an excellent, meaningful linkage between math and science.

Teaching sound physics and related topics presents an excellent opportunity to have students build a simple oscilloscope to examine and study the nature of sound waves.

You can make the oscilloscope using a can, a balloon, a small mirror and an inexpensive laser level. If you don't have a ring stand, you can improvise one usign wooden dowels inserted into pre-drilled holes in scrap lumber serve. Glue a small piece of mirror onto a balloon that is stretched over a metal cylinder made from a coffee can. Aim the laser at the mirror and direct the reflected laser beam onto a wall (as far away as possible as this will enlarge the pattern of vibrations that is observed and make for a more striking experiment).

When speaking into the can, the sound waves cause the membrane (and the mirror attached) to vibrate. The laser beam reflecting off the mirror also oscillates, allowing students to see a pattern of these vibrations on the wall.

If you are able to use a large darkened space, such as an auditorium or gymnasium, you may be able to produce a very large "web" of sound waves on a far wall. You may also "test" various sounds by inviting students to sing or play an instrument note into the can. Use a tuning fork to demonstrate visually the distinction between a pure tone and a combination of tones, such as the sound of the human voice. Also try your oscilloscope with infrasonic and ultrasonic sound waves by using dog whistles, cell phone tones or swinging a pool hose slowly, then quickly. For fun, place one in front of the speaker at a school dance for an inexpensive “light” show as the beam oscillates on the ceiling.

How do scientists know what an object looks like on the atomic level? This lesson lets kids understand how, by scaling up the atomic force microscope into a manipulative model, where students use a "probe" to discover what's hidden inside their box. This is a great activity for math and graphing as well. Ask the kids to take their plotted points home and figure out what their object is for homework.

This experiment teaches about the density of liquids, through a demonstration that kids find surprising. Fill two containers with two clear liquids - one water, the other isopropanol, and observe how an ice cube will float in one, and sink in the other. Don't tell your students that the liquids are different - let them discover for themselves.

If you drop a can of soda and one of diet soda into water, one will float and one will sink, because they have a different density. You can take this experiment a step further, by turning it into a richer lab investigation for your students.

If you're running an environmental science club or activity or if you have a green initiative in your school, you can ask your kids to look at principles of green engineering and do some experiments involving environmental science

Try out this scaled up variation on the classic "Egg in the Bottle" experiment. Light a match inside a large container (such as this old lava lamp), place a balloon on top and watch as it gets sucked in. Give your students some clues about why this happens and see if they can figure it out (hint: a pressure gradient is created). Then, once the balloon is inside the bottle, ask if they have a suggestion that will get it back out.