A farmer has a watermelon patch and upon inspection he discovers some of the local kids have been helping themselves to a feast. To foil the thieves, the farmer puts up a sign that reads, "Warning! One of these watermelons is poisonous - figure out which!"

The farmer returns to the watermelon patch the next day to discover none of the watermelons have been eaten, but finds another sign that reads, "Now two of these watermelons are poisonous - figure out which!"