Mr. Jones is driving past the state mental hospital when his left rear tire suffers a flat. While he is changing the tire, another car goes by, running over the hub cap in which he was keeping the lug nuts. The nuts are all knocked into a nearby storm drain. 

 

He is at a loss for what to do and is about to go call a cab when he hears a shout from behind the hospital fence, where one of the inmates has been watching the whole thing. 

"Hey, pal! Why don't you just take one lug nut off each of the other three wheels and use them to replace the missing ones? That'll hold your tires on until you can get to a garage or something." 

Mr. Jones is startled by the patient's seeming rationality, but realizes the plan will work, and installs the spare tire without incident. Before he leaves, he calls back to the patient. "You know, that was pretty sharp thinking. Why do they have you in there?" 

The patient smiles and says, "I'm in here because I'm crazy, not because I'm stupid."