Here's why:
* The moon is a large, rocky body in space. It doesn't produce its own light like a star does.
* The sun is a star, and it emits its own light and heat. This light travels through space and reaches the moon.
* The moon's surface reflects some of the sunlight that hits it. This reflected light is what we see from Earth.
That's why we see different phases of the moon: depending on the position of the sun, Earth, and moon, we see different amounts of reflected sunlight from the moon's surface.