Murphy was standing in front of the city hall when he saw a driverless car rolling slowly down the street.
He ran to the car, jumped in, and pulled on the emergency brake with a jerk.
Murphy got out and very proudly said to the man approaching him, “I stopped it!”
“I know, you idiot!” said the man. “I was pushing it!”
This joke is included in the Black Book of Jokes. You can order it from here.