Earth's mantle, the thick layer between the surface and core of our planet, is solid - but it is not rigid. And this certain flexibility that the mantle has allows it to convey heat more easily. The layers of the mantle closer to the hot core become pliable, almost like candle wax, and the heat rises in convection currents in the outer layers of the mantle, explaining why the deeper we dig (miners will contest), the hotter it gets.
That heat from the core, carried through convection currents to the surface, also stirs volcanoes and may be responsible for plate tectonic movements.
Humans didn't always know about this phenomena of how heat is transmitted from our earth's center to the surface. A scientist named Lord Kelvin who lived during the 1800s compared the earth to a baked potato that has been taken out of the oven, in the sense that earth started off hot and has been cooling ever since. Now we know that the potato metaphor doesn't work, because the planet isn't a solid lump through which heat diffuses at constant rate!