Hand-Held Heat Engine
You can make the liquid in this toy rise and fall in a cycle.
Challenge your students to get the liquid up to the top of the container without turning it over.

Your students may grab the hand boiler and squeeze it. The liquid will go up to the top; not because the students squeezed the boiler, but because the heat from their hands warms the gas, which pushes the liquid into the top chamber. After a little while, challenge your students to get the liquid back to the bottom of the boiler without turning it over. If they touch the top chamber with their hands, the liquid will go back to the bottom. The liquid will also go down eventually if you simply wait for the air in the bottom chamber to cool off. You may want to let your students try out ways to move the liquid by cooling the air rather than heating it.

This device highlights the interplay between temperature and pressure. In a closed container, as the temperature goes up, so does the pressure. As the temperature increases, the molecules of gas in the container move faster, which increases the pressure. As the pressure increases in one of the chambers, the liquid will be pushed into the other one. Conversely, as the air in one of the chambers cools off, the molecules of gas slow down, lowering the pressure. The liquid will move from an area of higher pressure to an area of lower pressure.