Why does the moon keep changing its shape?

This is because we can see only the parts of the moon that the sun shines on. When it is new moon, the unlit side faces Earth and we cannot see it. As the moon circles the Earth, more and more of the sunlit side becomes visible until we see the full moon. Then we see less and less of it and it eventually disappears. There are 29.5 days between one new moon and the next.