A man is strolling past the mental hospital and suddenly remembers an important meeting.
Unfortunately, his watch has stopped, and he cannot tell if he is late or not. Then, he notices a patient similarly strolling about within the hospital fence.
Calling out to the patient, the man says, "Pardon me, sir, but do you have the time?"
The patient calls back, "One moment!" and throws himself upon the ground, pulling out a short stick as he does. He pushes the stick into the ground, and, pulling out a carpenter's level, assures himself that the stick is vertical.
With a compass, the patient locates north and with a steel ruler, measures the precise length of the shadow cast by the stick.
Withdrawing a slide rule from his pocket, the patient calculates rapidly, then swiftly packs up all his tools and turns back to the pedestrian, saying, "It is now precisely 3:29 pm, provided today is August 16th, which I believe it is."
The man can't help but be impressed by this demonstration, and sets his watch accordingly.
Before he leaves, he says to the patient, "That was really quite remarkable, but tell me, what do you do on a cloudy day, or at night, when the stick casts no shadow?" The patient holds up his wrist and says, "I suppose I'd just look at my watch."