Explain the Density
Density is a measure of how densely packed matter is. One standard brick weighs more than the same size piece of Styrofoam but less than the same size brick of lead.
Example - How many kilograms of air are in your room? Assume your room is 10 m x10 m x 3.0 m. The density of air at sea level is about 1.2 kg/m^{3}.