One day, Benny the psychiatrist was on the subway coming home from work one day when he saw an elderly gentleman talking to himself and then laughing aloud. Every so often, the man would put up his hand, stop talking then start all over again.
Benny had to find out more.

“Excuse me, I hope you don’t mind me asking, but is there anything I can do to help?”
“Thank you but no. To keep me awake, I tell myself jokes when I’m travelling.”
“But why do you keep raising your hand?” asked Benny.
“Oh, that’s to stop me telling a joke I’ve heard before.”