What Causes the Phases of the Moon?

The moon changes shape because it moves around Earth, and sometimes we see more or less of its lit side. Imagine the moon is like a flashlight that's being turned on and off by the sun, depending on where the moon is in its path, we can see different parts of it.

The moon takes about 28 days to go all the way around Earth. As it moves, the part of the moon that faces us gets more or less lit by the sun. That's why sometimes we see a full moon and other times just a tiny crescent.

The sun is like the light source shining on the moon. When the moon is between Earth and the sun, we see a new moon, it looks dark because the lit side faces away from us. When the moon is on the opposite side of Earth from the sun, we see a full moon, the whole lit side faces us.