How Can We Find Out What's Going on Inside the Earth?

Two eggs in front of us and my question is what can you tell me about the inside of either or both of these eggs just by looking at them? You can't really tell anything. So if I were to put a challenge to you two as to how would you be able to find out what might be inside without breaking the egg open, what would you do? Maybe give it a shake and have a look at the other side. Admittedly that was a much better explanation than me smearing an orange with jam. However, I remain undeterred with my demonstrations because it's not just important to know about the surface of the earth but also what's going on inside of it too