A man is walking past a mental institution when he suddenly remembers an important meeting. As he does not have a watch he cannot tell if he is late or not. But he notices a patient nearby and called to him, "Pardon me, sir, but do you have the time?"
The patient answers, "One moment!" and throws himself on 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. Then with a compass, he 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 man, "It is now precisely 2:49 pm, provided today is April 26th, which I believe it is."
The man can't help but be impressed by this demonstration but at the same time, he was curious, "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."
