There were three guys stranded on a deserted island twenty miles from shore. One day the first guy decided to swim for it. He swam out five miles, got tired and drowned.

Later, the second guy opted to do the swim to the mainland. He made it out to ten miles, got tired, and drowned as well.

The last guy, who was a total idiot, also began to swim to shore. He swam until he was a half mile from shore, got tired, and swam the 19 and 1/2 miles back to the island.