Why does the moon seem to change shape each month?

The shape of the moon alters depending on the position of the sun in relation to the moon. The sun lights up only one side of the moon. As the moon orbits the earth, we see this lit up area from different angles. When the sun is almost behind the earth, it lights up the whole of the moon (this is called a full moon). When the sun is off to one side, part of the moon is in deep shadow and all we can see is a thin slice, or crescent, of the moon's lit surface.