The moon is Earth's closest neighbor in space. It orbits (circles around) the Earth about once every 29.5 days. The moon does not produce its own light -- it shines because sunlight reflects off its surface. As the moon orbits Earth, we see different amounts of its sunlit side, creating the changing shapes we call phases: new moon (invisible), crescent, quarter, gibbous, and full moon. The phases follow the same pattern every month.
Track the moon's shape every night for a month using a moon journal. Use a ball (moon) and flashlight (sun) in a dark room, moving the ball around the student's head (Earth) to show how the lit portion changes depending on the angle. Teach the phase names in order: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, waning crescent, and back to new moon.
Look up at the sky at night and you will probably see the moon. Sometimes it is a big, bright, round circle. Sometimes it is a thin curved sliver. Sometimes you cannot see it at all. These changing shapes are called phases, and they happen like clockwork every month. But the moon is not actually changing shape -- what changes is how much of its sunlit side we can see from Earth.
Here is the key fact: the moon does not produce its own light. It shines because sunlight bounces off its surface and reaches our eyes. At any given moment, exactly half of the moon is lit by the sun (the half facing the sun) and the other half is in darkness. As the moon orbits Earth over about 29.5 days, our viewing angle shifts, and we see different portions of that lit half.
When the moon is between Earth and the sun, the sunlit side faces completely away from us. We see only the dark side -- this is a new moon, and the moon is nearly invisible. As the moon moves along its orbit, a thin sliver of the sunlit side comes into view: a crescent. A week later, we can see exactly half of the sunlit side: a quarter moon (it is called "quarter" because the moon is a quarter of the way through its cycle). The visible portion keeps growing (waxing) through the gibbous phase until we see the entire sunlit side: a full moon. Then the process reverses: the visible portion shrinks (waning) through gibbous, quarter, and crescent phases until we are back to a new moon.
This cycle repeats every month, as reliable as clockwork. If you track the moon for a few weeks, you can predict what it will look like on any given night. One more thing that surprises many people: the moon is often visible during the day, too. It is not a nighttime-only object. During the crescent and quarter phases especially, you can often spot the moon in the blue daytime sky if you look carefully.