At Creation, G-d made the "two great lights," the sun and the moon.

The sun is constant; every day the same big fiery ball appears in the sky above us. But the moon is always going through changes. It has its ups and downs; one day it is full, then it wanes, getting smaller and smaller. Yet even after it seems to have completely disappeared, it renews itself, and grows again.

Even as we grow and change, we must retain our consistency.