Once upon a time, there was an evil physician in King Arthur's Court. When Merlin went off one day to gather bat's wings and dragon whiskers, the physician snuck into Merlin's room in the castle and stole his book of magic.
Using the blackest evil spells he could find in it, he terrorized the kingdom. Even the King was afraid of him.
A royal Page saw Merlin returning and went out to warn him. He and Merlin used a secret entrance into the castle and Merlin made his way to the royal dining room, where everyone was eating in silence, afraid that anything they said might offend the powerful magic of the physician. Merlin cast a spell from behind the curtains that turned the evil physician into a normal-looking apple. Everyone was stunned.
Then Merlin stepped out and everyone realized what he had done. Merlin bowed to their applause, then warned them that no spell lasts forever. If the spell started wearing off, the apple would start to grow bigger and heavier, until it turned back into the evil physician. If that started happening, Merlin was to be summoned.
The King thought a momment, then issued a decree: the apple was to be kept in the royal counting room, where it could be put on the royal scales and weighed each day. In other words,
A weigh a day keeps the doctor an apple.