He discovered a flat tyre when he was about to go home.
He jacked up the truck and took the flat tyre down.
When he was about to fix the spare tyre, he accidentally dropped all the bolts into the drain.
As he can't fish the bolts out, he started to panic.
A patient happened to walk past and asked the driver what happened.
The driver thought to himself, since there's nothing much he can do; he told the patient the whole incident.
The patient laughed at him & said "can't even fix such a simple problem... no wonder youare destined to be a truck driver..."
"Here's what you can do, take one bolt each from the other 3 tyres and fix it onto this tyre. Then drive to the nearest workshop and replace the missing ones, easy as that"
The driver was very impressed and asked:"You're so smart but why are you here in a Mental Hospital?"
"Hello, I am here because I'm CRAZY, not STUPID!"