Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to firstname.lastname@example.org. How does the inside of the Earth stay boiling hot for billions of years? Henry, age 11, Somerville, MassachusettsOur Earth is like an onion, with layers of crust, mantle, outer core and inner core. The deeper you go, the hotter it gets – parts of the core can be as hot as the surface of the Sun! As a professor of earth and planetary sciences, I study the insides of our world and use seismic waves (sound waves produced by earthquakes) to image the Earth’s internal structures. The lithosphere, which is the upper part of the mantle and the crust, is about 60 miles thick. It’s divided into several large blocks called plates – like puzzle pieces that fit together and cover the Earth’s surface. Plate movement is essential for the evolution of life on Earth as it changes the environment and forces life to adapt. Plate motion requires a hot mantle, and temperatures at the bottom of the plates are around 2,400 degrees Fahrenheit. By the time you get to the inner core, the temperature is nearly 10,800 F – as hot as the surface of the Sun! The heat comes from the Earth’s formation 4.5 billion years ago and the decay of radioactive isotopes, which release energy that converts to heat. Without the Earth’s internal heat, the plates wouldn’t move, the Earth would cool down and our world would likely be uninhabitable. Next time you feel the Earth under your feet, think about that!