How Can We Find Out What the Earth Is Made Of?

"I'm pretty sure the Earth isn't hollow inside, it might be wrong. So we have two eggs in front of us, and my question to the pair of you is what can you tell me about the inside of either or both of these eggs just by looking at them?" "If you were to shake this egg here, nothing. Nothing. However, were you to shake this Egg Here? It's not hollow. I guess they're both roughly spherical, and well, that one looks like it's hollow inside," he said.