system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: if the deepest depth drilled by man is about 8 miles, and the crust is nearly 20 miles deep, how were scientists able to discover that there is an upper and lower mantel and inner and outer core?
gpt
The same way you are able to tell what's in the box your grandmother sent you at Christmas.  When you shake it, a sweater sounds different from a PS4 controller.  Obviously scientists can't shake the earth, but the earth shakes itself sometimes, and scientists in different places are always listening (or rather their seismographs are listening).  By comparing what different locations record, they can make good guesses about what's inside, just like you may be able to do.  

Edit: Thanks for the gold!