There was a farmer who raised watermelons. He was disturbed by some local kids who would sneak into his watermelon patch at night and eat watermelons.
After some careful thought he came up with a clever idea that he thought would scare the kids away for sure.

So he made up the sign and posted it in the field. The next day the kids show up and they see this sign, it says "Warning!! One of the watermelons in this field has been injected with cyanide."

So the kids run off, make up their own sign and post it next to the sign that the farmer made. The farmer shows up the next week and when he looks over the field he notices that no watermelons are missing but he notices a new sign next to his.

He drives up to the sign which read: "Now there are two".