A mathematician and an engineer are stranded on a desert island. Along with them, are several boxes of canned food. Try as they might, they were unable to bust open of the cans with the rocks that were available. Finally, the engineer decides to explore the rest of the island to try to find food or something to help open the cans.
Several hours later the engineer arrives back at the camp sight to find the mathematician eating a can of peaches, with several empty cans lying around.
"How did you open the cans?" asks the befuddled engineer.
"Easy," says the mathematician. "First, I assumed there was a can opener..."