Well, we spent quite a while learning about earthquakes. And boy, was I ever surprised. Okay, there are actually TWO types of earthquake waves, and they each have their own characteristics. First, there are the fastest, or Primary waves. These waves of energy compact and expand the earth. They can also move through water. Have you ever squished a Slinky and pulled it apart again? Those are the Primary Waves. Then there are the slower waves: Secondary waves. These shake the earth back and forth. However, these waves won't go through water. Just disappear! They need those links solids have to transfer their energy. So to tie that in to Earth Science, we learned how the S waves won't go through the liquid outer core, but the P waves will. That's how scientists knw about the core.