Density is a property of all objects and is simply the mass of the object divided by the object's volume. As a concrete example, compare the densities of a tennis ball and a golf ball. A golf ball is clearly smaller than a tennis ball, and it turns out a golf ball is slightly heavier than a tennis ball. The end result is that a golf ball is denser than a tennis ball. Another important topic is relative density .
Relative density is just the density of some object relative to some other thing or substance.
The answers are sink, float, and relative density. Mystery of the Missing Iron Now to solve the Mystery of the Missing Iron. The planetesimals from which Earth formed were pretty much homogeneous bodies. Light and dense materials were intermingled in them, and since the rocks were solid, these materials had no way to move around based on relative density. The result of this is that as the early Earth was being formed, the stuff from which it was born was pretty well mixed up.