Why is the Moon not always completely round ?

The Sun always lights up half of the Moon’s surface the part facing the Sun. But, as the Moon moves around the Earth and the Earth moves around the Sun, different parts of the sunlit Moon are visible to us. Sometimes we see the whole of the sunlit surface, this is what we call the ‘full’ Moon. Sometime we see only the left or right part, which looks like as slice of melon. So we see the Moon in several stages. These are called phases of the Moon and are known as: full moon, warning Moon, new Moon and waxing Moon.