How do we know that the inside of the earth is hot? The best evidence are volcanoes, geysers (like the ones at Yosemite and Yellowstone parks), and earthquakes (tectonic plates). The heat is there, the tricky part is moving it and changing it into energy we can use. The science of thermodynamics is how we figure those things out.

Think about the two words "temperature" and "energy." While they seem different, it turns out these two are very much the same. Geothermal energy comes from the high temperatures that are below the surface of the earth. You can think of the hot insides of the earth as places with lots of heat energy, because they are the same. We absorb some of the energy from inside the earth and bring it back up to the surface so we can use it. So if we can bring heat energy up to the surface, does that mean we can power your light bulb? Not yet! We need to convert the heat energy into electric energy. This is done with turbines, like the ones on big planes at the airport. The turbine turns an electricity generator, which gives electricity to power your light bulb. California is home to the largest geothermal power field in the world called The Geysers. Typically homes do not have a geothermal power plant, but some homes can still use heat from the earth to reduce power use from the electric company. This is called (not surprisingly) geothermal heating. You pump cold water deep into the ground, and it comes back heated. This is not a good option for everyone, and it depends a lot on the geography and climate of your home. Thanks for the great question!