There was a farmer who raised watermelons. He was doing pretty well but he was disturbed by some local kids who would sneak into his watermelon patch at night and eat his watermelons. 

After some careful thought he comes up with a clever idea that he thinks will scare the kids away for sure. So he makes up a sign and posts it in the field. The next day the kids show up and they see this sign, which says, "Warning, one of the watermelons in this field has been injected with cyanide." So the kids run off and make up their own sign, which they post next to the sign that the farmer made. The farmer shows up the next day to look over his field. He notices that no watermelons are missing but he notices a new sign next to his. He drives over to the sign and takes a look. 

It says, "Now there are two"