Here's why:
* Full Moon: The entire face of the moon is illuminated by the sun.
* Waning Gibbous: The moon is still more than half illuminated, but the lit portion is slowly shrinking, like a "gibbous" (hump-shaped) cookie that is being eaten away.
Think of it like this:
* Full Moon: You have a whole cookie.
* Waning Gibbous: You've taken a few bites, but there's still a lot left.
The moon continues to "wane" (shrink) until it reaches a new moon, where it is completely dark.
