Why Is the Moon Sometimes Round and Sometimes Not?

The moon, like the earth, is round all the time. Why then does the moon we see everyday change its shape? The moon does not shine by itself. It is the sun-light that illuminates the moon so that we can see it. As the moon keeps revolving around the Earth, we will @ a round moon when the whole moon faces us. But, if only half of the moon faces us, we see half of the moon. Some-times we see a crescent moon. That means only that little part of the moon is facing us.