If we want to give out 10 sweets equally among 5 children, we divide 10 by 5. The quotient is the number of sweets which correspond to each child. It is written "2 sweets/child", and you read it "two sweets per child".

In the same way, if we want to equally distribute the 1740 g of the mass of an iron ball among its 200 cm³ of volume, we divide 1740 by 200. The quotient obtained - the density- represents the grams which correspond to each cubic centimetre of the ball, and gives us an idea of how concentrated the matter is. In this case 8.7 g/cm³.

The cubes presented below are equal and have therefore the same volume, but one contains a different mass from the other.

The balls in both cubes are equal and represent atoms. If these two showed the possibilities of the atoms in a substance to group together, in which cube would the matter be more concentrated? Obviously in the cube on the left, where the same volume contains more matter, and is therefore more dense than the cube on the right.