You don’t have to watch a covered saucepan to know when the food or water inside it is boiling. The lid will begin to bounce up and down when the water begins to boil. The push that moves the lid comes from steam.
As the water inside the saucepan is heated, the molecules take up energy. That energy makes the molecules speed up and push hard against one another. When the water is hot enough, some of it changes from a liquid to a gas called steam.
The molecules of gases move around more and are further apart than the molecules of liquids. So the steam expands. It takes up much more space than the hot water did. But there is only one place where the steam can escape – through the spaces around the saucepan lid. It squeezes out with a hard push, and the push bounces the lid up and down.
Steam also can be used to push the moving parts of machines—to run ships, trains, and factories that make electricity. Such big machines need very strong pushes from expanding steam. So huge amounts of water must be boiled.
Picture Credit : Google