Watermelon
A farmer in the country has a watermelon patch and upon inspection he discovers that some of the local kids have been helping themselves to a feast.

The farmer thinks of ways to discourage this profit eating situation. So he puts up a sign that reads "WARNING; ONE OF THESE WATERMELONS CONTAINS CYANIDE!

The farmer returns a week later to discover that none of the watermelons have been eaten, but finds another sign that reads,


          "NOW THERE ARE TWO!"