system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how do scientists determine the density of something like a planet or moon?
gpt
Density is the amount of mass in relation to the volume of the object. They can tell the volume of the object by looking at it ("you can tell by the way it is"), and the mass can be deduced by looking at the orbital distance and speed, and how it impacts other objects of better known mass.