We gave each student a small ball on a stick (golf ball or ping pong ball) to represent the moon. We turned out the classroom lights and used a lamp to represent the sun. First, everyone held their ball so that it was eclipsing the lamp. I asked, “What is the angle between your arm when pointing one arm at the lamp and holding the ball in the new 'moon' position?” 0 degrees.
As we moved through the moon phases, the students observed the angle relationship created by the “sun” and “moon”:
We used the term acute, right, obtuse, and straight to describe the angles formed.