This exercise has students use a simple physical model of the Earth, sun, and moon to understand why the moon changes phases from the perspective of Earthly observers. Students hold up balls representing the moon in a room with a single light source (representing the sun). As they turn relative to the sun, holding the "moon" out in front of them, they will see changes in how much of the side that they can see is lit. At the same time, they can check and see that overall, half of the "moon" is lit no matter what position it is in relative to them. On this Starting Point page, users can access information about the exercise's learning goals, context for use, teaching notes and tips, teaching materials, assessment ideas, references and topics covered.