Boyler

Meet Boyler, a syringe full of gas who pushes back harder the more you squeeze him.

Gas pressure comes from countless molecules hitting the walls of their container. Squash the gas into a smaller space and each molecule hits the walls more often, so the pressure rises. If the temperature stays the same, the molecules keep the same average speed, and only the crowding changes.

Boyle's law says p₁V₁ = p₂V₂ for a fixed amount of gas at constant temperature. Seal 60 mL of air at 100 kPa in a syringe and push the plunger in to 20 mL. The volume is a third, so the pressure triples to 300 kPa.

Pull the plunger out to 120 mL and the pressure halves to 50 kPa. Squeezing quickly warms the gas a little, so the law holds best when you push slowly.

Robert Boyle published this result in 1662. Divers rely on it, because the air in their lungs shrinks and expands as the water pressure changes.