John, Brian, and Bill were all locked away in a mental institution for many, many years. One day, the head doctor tells them that if they pass a test he wants to administer to them, they'd be deemed mentally competent and free to leave the nut house. Should they fail, however, they'd be locked away for another five years of observation. All three took the doc up on his offer. The four of them went to the hospital's indoor pool. The pool was drained of water. The doctor leads the patients up to the diving platform, some 60 feet in the air. The doctor motions to John. "Jump." Without hesitation, John leaps off the platform, right into the pool, breaking both arms in the process. The doctor notes this on his clipboard and tells Brian, "Jump." Also without hesitation, Brian flies off the platform into the empty pool, breaking both of his legs. After noting the results on his clipboard, the doctor tells Bill, "Jump." Bill shakes his head. "Naw. I don't think so." The doctor notes this on his clipboard rather enthusiastically and says, "Congratulations, Bill. You're a free man. Just tell me one thing. Why didn't you jump?" "Easy," Bill says, "I can't swim."