The Earth's gravitational field is pulling on air, and this pull, or "pressure" of air, is called atmospheric pressure. If air is given more heat, this is extra energy for its molecules. If they have more energy, they can move further apart, so there will be fewer molecules occupying the same space and the air pressure will decrease.

Experiment: Try this out!

We suggest this experiment be done by teacher/supervisor only, with the children in viewing distance only.

Put a small amount of water in a glass bottle with a long narrow neck and heat it just to boiling. Remove the bottle from the heat. Place on a hard stable surface. Immediately place a peeled hard boiled egg on the mouth of the bottle. The vaporized water displaces some of the air in the bottle and condenses. The vapor begins to cool, thereby lowering the pressure. Watch what happens. The heated water in the bottle heats the air, exciting the air molecules and raising the pressure inside the bottle. Because the egg is just sitting on the top it is lifted enough by the higher pressure inside so some air escapes to equalize the pressure. After the water cools, the air cools back down and the pressure inside drops. The pressure outside presses the egg against the bottle and make a seal. Once the air inside the bottle cools enough and the pressure inside drops enough, the now higher pressure outside pushes the egg through the bottle neck.