"Doctor. . .I haven't been able to sleep for a week!"
"Really!" exclaimed the Doctor "Do you know what is causing it?"
"Well. . . .Every night I dream the same dream. I dream about a door with a sign on it.
"Really," the Doctor said thoughtfully.
"Yes," the man replied. "Every night I am standing in front of this door with a sign on it. I push and I push and I push- - - but I still can't open it!"
"Well," the Doctor seemed a little puzzled. "What exactly does the sign say?"