"I have to light the other side of the world. When it's night here, it's day there. But I have an idea. Watch the sky tonight. There will be a surprise."
The animals waited impatiently all day long. When night fell, they saw a tiny crescent of light in the sky.
The next morning the sun asked the animals how they liked the surprise.
The animals soon became used to having light at night. They didn't marvel at it anymore and they didn't even think of thanking the sun.
This made the sun angry. He made the moon smaller, until one night it completely disappeard.
The next day they went to apologize to the sun.
Ever since then, the moon disappears completely one night a month. But it always returns -sometimes as just a tiny crescent, sometimes as a round full moon- to light the night for the animals.