The moon doesn't really change shape, it is always a full, whole, spherical moon. We just can't always see all of it, we can only see the light side. The sun shines on a specific side of the moon, and because earth (and the moon) are always moving, the sun is shining on different areas of the moon every day. During a new moon, the sun is shining on the part of the moon we can't see.
the earth blocks it from the sun
The revolving of the earth and the rotating of the moon.
wax and wane
rise set wax wane
The moon must wane in order to change from full to third quarter.
The moon waxes and wanes due to the shadow of the earth. The shadow of the earth passes between the sun and the moon casting a shadow on the moon and making it appear in different shapes.
During the time at sea, the sailors saw the moon wax and wane three times.
Wax and wane
Two weeks, and two weeks to wane.
The shadow of the earth on the moon.
The opposite of to wax is to wane. Wax means to grow and wane means to diminish