Why does the Moon's shape change every night, and why does it sometimes look fully round?

The Moon does not make its own light. We can see the Moon because sunlight hits it and bounces back to our eyes. As the Moon travels around the Earth, its position next to the Sun changes a little each day. That is why sometimes we see the whole bright part, and sometimes we only see a slice of it.

This trip around the Earth takes about 29 days. During that time, we first see a small crescent. Then the Moon slowly grows bigger and bigger, until one night it looks fully round — that is called a full moon. After that, it slowly gets smaller again and turns back into a crescent.

When the Moon looks fully round, that is, when it is a full moon, the Earth sits between the Sun and the Moon. The whole side of the Moon facing us is lit up by sunlight. That is when we see a big, bright, round Moon in the sky.