A watermelon farmer was determined to scare off the local kids who went into his watermelon patch every night to eat their fill.

After some thought, he made a sign that said, "Warning! One of these watermelons has been injected with Poison!"

He smiled smugly as he watched the kids run off the next night without eating any of his melons.

A week later, the farmer was surveying his field. To his satisfaction, no watermelons were missing, but a sign next to his read:
NOW THERE ARE TWO!