The moon transforms from being completely yellow, forms little slivers and disappears altogether in twenty-eight days. The reason this happens is because there are tiny lights on the moon that makes the moon shine bright.
One by one, these lights get dirty from all the moon dust, causing us to only see slivers of the moon. But, every twenty-eight days, a man flies up to the moon to clean the little lights. It takes him a long time to finish the process of cleaning the tiny lights, enabling us —again— only to see slivers of the moon.
When the man is finished, we can see the entire moon, which we call a full moon.