Why does the moon change its shape?

The moon, which causes high and low tides on the Earth, is a sphere. It does not change its shape; only our view of it changes. The reason is that unlike the stars, the moon does not produce light, but only reflects the light of the sun. Since the moon goes around the Earth once in a month, the orientation of the moon, the sun, and the Earth respective to one another changes. 

For this reason, the illuminated part of the moon cannot always be seen clearly from the Earth. Sometimes we see it full, sometimes half, and sometimes not at all. The side of the moon that faces us remains the same. We can see the craters and dark spots on the moon, which are known as 'mares'.