Pressure
A pushing force spread across an area. Gas trapped inside a volcano pushes on the rocks around it — and if that pressure gets high enough, something has to give, and the volcano erupts.
Pulley
A wheel with a groove for a rope or cable. A pulley lets you change the direction of a pull — and if you combine several pulleys, you can lift heavy things with much less force than their weight.
Pumice
A light, rocky foam made when gas-filled lava cools quickly. Pumice is so full of tiny bubbles that some of it actually floats on water — the only rock that does.
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