Hand-Held Heat Engine
Can you get the liquid in a so-called "hand boiler" to travel up and then back down again without turning it over?
Pick up the hand boiler and gently squeeze it. As you hold it, the liquid will travel up to the top—not because you squeezed the boiler, but because the heat from your hand warms the gas inside, which pushes the liquid into the top chamber.
After a little while, try to get the liquid back to the bottom of the boiler without turning it over. If you touch the top chamber with your hands, the liquid will travel back down to the bottom. It will also travel down eventually if you simply wait for the air in the bottom chamber to cool off. You may want to try out ways to move the liquid by cooling the air rather than heating it.
A hand boiler highlights the interplay between temperature and pressure.
In a closed container, as the temperature rises, 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 chamber. Conversely, as the air in one of the chambers cools off, the moving molecules of gas slow down, lowering the pressure. The liquid will move from an area of higher pressure to an area of lower pressure.