A police officer had just pulled over a car full of nuns for going too slow on a major highway. The conversation went like this：
Police officer： “Why were you driving so slowly？”
Nun driver： “I kept seeing all these signs with the number 20 on them and figured that was the speed limit.”
Police officer： “No， Sister， that is the highway number.”
Nun： “Oh， I'm so sorry， Officer， I didn't know that.”
Then the police officer looks into the back seat and sees that the nuns there have panic-stricken faces and white knuckles from holding on to each other too tightly.
Police officer： “What's wrong with the nuns in the back seat？”
Nun： “Oh， we just got off Highway 101 a few miles ago.”