Prof. Sabine Stanley, author of "What's Hidden Inside Planets," tells us how the interior of Earth helps make this planet a nice place to live. She tells us that the Earth's mantle is actually solid rock, not liquid rock. She also explains what the Earth's interior and birthday balloons have in common. Spacepod listeners can get a 30% discount on "What's Hidden Inside Planets" with the code HPLAN if you order through Hopkins Press at press.jhu.edu. (This code is shared as a courtesy, Spacepod does not receive compensation if you order a book.)