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What is heat?

Boiling water at a hot spring.Boiling water at a hot spring.Heat is the energy an object has when its molecules are moving or vibrating (shaking). The hotter the object is, the more its molecules move. In solids, the molecules may vibrate so much they break apart. The solid has melted to become a liquid. Heated further, the molecules spread out: the liquid turns to gas. Temperature is a measure of how fast the molecules in an object are vibrating.

Lava is hot, molten rock from inside the EarthLava is hot, molten rock from inside the Earth

Where does heat come from?

Nearly all the Earth’s heat comes from the Sun. It travels to us by radiation (see below). But there is also a source of heat inside the Earth. Called geothermal energy, it comes from the molten rocks deep below ground. This molten rock sometimes erupts at the surface through volcanoes all over the world.


Water is an unusual substance: it expands both as it heats up (turning to water vapour) and as it cools down (turning to ice).

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