Model and explain the cycle of moon phases as a result of the changing angle from which the sunlit moon is seen from Earth.
The moon does not produce its own light; it shines because sunlight reflects off its surface, the same way a mirror or a ball in sunlight would. As the moon orbits Earth over about 29 days, the angle between the sun, the moon, and Earth changes, so we see different amounts of the moon's sunlit half from our position on Earth. That changing view is what creates the phases.
At new moon, the sunlit half faces away from Earth, so we barely see it. At full moon, the sunlit half faces directly toward Earth, so we see the whole lit face. In between, we see a growing or shrinking sliver or half of the lit portion, depending on the moon's position in its orbit.
Mastery looks like: They can model the moon phase cycle and explain that phases result from the changing angle of sunlight reflecting off the moon, not the moon's shape changing.
Common stumbles: Thinking Earth's shadow causes regular moon phases, which actually describes a lunar eclipse, a separate and rarer event; believing the moon's actual shape changes.