This activity is frequently used to show the causes of the lunar phases. Generally a teacher tells students how to position the ball to show a full moon, a crescent moon, etc. This particular lesson is a little different in that the students have to figure out on their own how the balls need to be positioned. This lesson makes a good follow-up to a lesson about the motions of the Earth, Moon and Sun (such as PUMAS Example 03_10_04_1), but it can also stand as an independent lesson.