Question

why does the moon appear to change its shape

Answer 1

because earth rotates

Answer 2

The earth revolves around the sun. The moon reflects the light from the sun. When the moon is between Earth and Sun, the lit side is hidden (no moon). The moon moves around the earth as it has to reflect light from the sun. As it moves, the lit side comes to view to us. This is the reason we can see the moon change it's shape.