system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we know the different layers of the earth? (Mantle & Core) Their composition and general temperature range?
gpt
If you post this in askscience you will get a better answer.  But in ELI5 you get dummies like me.  They developed sonar and seismic  meters.   But essentially they shot different types of rays into the earth and wrote down what came back.   Then they compared the different results.  Based on the data they developed a theory.  

Thats it!