 Here's a schematic diagram of a cross section of the earth. You can see this tiny thin part here is the crest. From the base of the crest, all the way to this boundary is the mantle. And from this boundary all the way to the center of the earth is called the core. We also distinguish between the upper and lower mantle, that's this boundary right here, and between the inner and outer core. That's this boundary right here. So how do we decide where to actually place these boundaries? Well, that knowledge comes from observations of seismic waves. And in fact, we can use observations of seismic waves to tell us a lot of other things about the composition of the interior of the earth, such as its exact mineral agit composition, and the pressure, and the density, and the temperature.