A witch doctor kept the members of his tribe in virtual subjugation by means of his powerful magic. Whenever one of the tribespeople tried to revolt against the witch doctor, the tyrant would utter a magic incantation and turn that person into an apple.

One night a group of the doctor's desperate subjects sneaked into his hut, opened his book of magic recipes, and learned the apple incantation. When the doctor awoke, the people turned him into an apple.

But the magic book informed the people that, if the apple ever dried out and changed significantly in weight, the apple would change back into a doctor, who would take his revenge.

So every day they would place the apple on a scale to make sure that its weight remained the same.

And, of course, the moral to the story is:

A weigh a day keeps the doctor an apple.