The moon doesn't actually change its shape. The apparent change in the moon's shape, which we call the phases of the moon, is caused by the changing angles of sunlight reflecting off its surface as it orbits the Earth.

Here's how it works:

1. The Moon's Orbit: The moon orbits the Earth, taking roughly 29.5 days to complete one full cycle.

2. Sunlight and Reflection: The sun illuminates the moon, and we see the portion of the moon that is reflecting sunlight.

3. Changing Angles: As the moon orbits Earth, the angle at which sunlight hits the moon changes. This means we see different amounts of the illuminated surface.

4. The Phases: The different shapes we see are called the phases of the moon: New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, and Waning Crescent.

So, the moon's shape remains constant, but the way we see it changes due to the changing angle of sunlight reflecting off its surface.