A motorist happened to have a flat tyre outside the compound of a mental hospital. He was changing the tyre when he accidentally pushed the four fastening nuts into the drain gutter and could not find them anymore. He was at his wit's end, not knowing what to do. 

An inmate having his break inside the mental hospital compound saw what happened through the fence. He grinned and advised the man to take one nut each from the other three wheels to fasten the one with the lost nuts. The problem was thus easily solved. Having done the job, the motorist thanked the mental hospital inmate and asked him, "How come you are in there?"

The inmate grinned broadly and replied, "I am in here because I am crazy, not because I am stupid !"