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.
Do you have any recommended science experiments using inexpensive materials? Did you find these ones useful? Let us know what you think.