Here's why:

* The moon's phases are caused by its position relative to the sun and Earth. As the moon orbits the Earth, the angle at which the sun's light hits the moon changes.

* At full moon, the Earth is positioned directly between the sun and the moon. This means the entire side of the moon facing us is bathed in sunlight.

* We only see the illuminated portion of the moon. When the entire illuminated side is facing us, we see the full moon.

Think of it like this: imagine holding a ball in your hand and shining a flashlight on it. If the flashlight is directly behind you, you'll see the entire illuminated side of the ball. That's similar to the full moon!