A witch doctor kept the members of his tribe in subjugation by means of his powerful magic. Whenever one of the tribe's people tried to revolt against the tyrant, he uttered a magic incantation and turned the person into an apple.
One night, a group of the witch doctor's subjects sneaked into his hut, opened his book of magic and learned the apple incantation. When the doctor awoke, the people recited the magic phrases and turned him into an apple.
But the magic book warned that if the apple ever dried out and changed significantly in weight, it would change back to the person under the spell. Not wishing for the doctor's revenge, every day the tribe's people would place the apple on a scale to make sure its weight remained the same.
Moral: A weigh a day keeps the doctor an apple.