Density is a comparison between the mass and volume of a substance. Mass is the amount of matter in a substance and volume is the amount of space something takes up. We know that the different layers of the Earth (the crust, mantle, and core) have different densities. The density of the crust varies from 2.7 g/cm³ to 3.0 g/cm³. The density of the mantle is 3.3 g/cm³ to 5.7 g/cm³.
To understand the layers of the Earth better we will investigate how materials with different densities interact with each other.
View this video for a little more information about why the Earth has layers: