system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are we able to know what the core of the Earth is made of if when we haven't even drilled past the crust?
gpt
We get information on the interior from a few different places.

One basic source is from volcanic material--eruptions can bring material from the mantle up to the surface, and examining that is one way we learn about the planet.

For the core itself, we can learn from analyzing the waves sent through the earth when an earthquake happens. 

The more dense a material is, the faster waves travel through it. So if an earthquake happens in Japan, stations around the world might notice the waves from that earthquake. But they all record them at different times--measuring the time it took the waves from the earthquake to reach different recording stations can tell us how fast the waves were traveling, and therefore how dense the material is.

Knowing this helps us determine what kind of material is in the core, by comparing its density to what materials we know, and what materials are likely to be found in the earth.