Three patients in a mental institution prepare for an examination given by the head doctor.
If the patients pass the exam, they will be free to leave the hospital. However, if they fail, the institution will detain them for seven years. The doctor takes the three patients to the top of a diving board overlooking an empty swimming pool, and asks the first patient to jump. The first patient jumps head first into the pool and breaks both arms. Then the second patient jumps and breaks both legs.
The third patient looks over the side and refuses to jump. "Congratulations! You're a free man. Just tell me why didn't you jump?" asked the doctor. To which the third patient answered, "Well Doc, I can't swim!"
