Why Does the Moon Have Phases?

The Moon is the brightest object in the night sky when its whole face is showing. This only happens at full Moon. So why isn't it always a full Moon? 

The Moon is constantly orbiting the Earth. We only see the part of the Moon that is lit up by the Sun. During its orbit, different amounts of the lit-up side of the Moon are visible from Earth. This makes it look like the Moon is changing shape. We call these changes the phases of the Moon

It takes 29.5 days to go from one full Moon to the next. We call this a lunar month or a lunar phase cycle.