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13 ? What causes the Moon to go through phases
The moon, Earth, and sun are in constant motion--the moon around the Earth and the Earth around the sun. When we face the sun, we see light. When we don't face the sun, we see darkness

Add the moon into this: the moon does not have its own light--it only reflects the sun. So like Earth, it has a face of light and darkness based on the sun's position relative to it. The amount of light we see "emitting" from the moon from our vantage point on Earth is based on the moon's relative position to the sun, which is in a constant cycle. So every 29.5 days (a lunar month), we see the same phases.



Imagine a soccer ball hung in the middle of a dark room at about the level of your head. Now imagine a flashlight shining on it from the side. Half is lit up, and half is in shadow. Now imagine walking around the soccer ball. When the ball is between you and the flashlight you can only see the shaded side. When you are between the ball and the flashlight the whole ball appears to be lit up.
The soccer ball is the moon. The flashlight is the sun. You are the Earth. Of course the moon goes round the earth, but the principle is the same.
Because of the way it orbits and the angle at which it is pointing towards the sun