Energy of Motion

How does this increase in pressure produce energy of motion?

Think of it like this: Suppose you boil water in a covered pot. The boiling water changes to steam, a gas, which takes up more room than the liquid. So the pressure increases. If the cover is not attached tightly to the pot, the increased pressure forces it up. You can see the cover on a pot of boiling water move upwards. The rattling of the lid indicates an intermittent release of pressure.

When a piston in an engine is pushed up into its cylinder, the upward movement traps the mixture of air and fuel above it and compresses it to one-eighth or one-ninth of its original volume. This puts the molecules closer together, so they hit the piston head far more often. Thus, the pressure goes up.