Consider the following discussion between two students about the cause of the phases of the moon.

As the moon orbits the Earth, we can see a different amount of the moon is lit by the sun from our perspective on Earth. Sometimes the moon is completely lit, and other times it is completely dark.

The illuminated shape of the moon that is visible from Earth is called a phase. When you cannot see the moon at all, this phase is called a new moon. Several days after a new moon, we see the moon as a thin sliver of light called the crescent moon. As the moon appears larger and larger, it becomes a quarter moon and gibbous moon. A full moon is visible when the whole side of the moon facing Earth is completely illuminated by the sun. On a clear night, a full moon looks very bright.