The moon doesn't actually change shape. What we see as the different "shapes" of the moon (the lunar phases) are actually caused by the changing angles of sunlight reflecting off the moon's surface as it orbits the Earth.

Here's a breakdown:

* The moon always has the same shape: It's a sphere.

* Sunlight illuminates the moon: Just like the Earth, the moon is illuminated by the sun.

* The moon orbits the Earth: As the moon orbits, the angle at which sunlight hits it changes.

* We see different amounts of the illuminated side: This is what we perceive as the lunar phases, from a sliver of light (new moon) to a full circle (full moon).

Think of it like holding a ball in front of a lamp. As you rotate the ball, different parts of the ball are illuminated.