ELI5: Why does the moon have phases?

The Moon has phases because as it circles around the Earth, we see different amounts of its sunlit side.

The Moon does not make its own light; it is a giant, dark rock that acts like a mirror reflecting the bright Sun. Even though half of the Moon is always lit up by the Sun, we cannot always see all of that lit-up half from where we stand on Earth.

You can easily understand this with something from your everyday life. Imagine standing in a dark room holding a basketball at arm's length, while a friend shines a bright flashlight at you from across the room:

• If you hold the ball right between your face and the flashlight, the side facing you is dark. This is like a new moon, when the Moon seems to disappear from the night sky.
• If you slowly turn around while still holding the ball, you start to see a thin sliver of light on the edge. This is a crescent moon.
• When you turn so the flashlight is behind your back, the light shines directly on the ball in front of you. You can see the whole lit-up side. This is like a full moon.
• As you keep turning around in a full circle, the light shrinks again until the ball goes dark.