One day, a blonde and a brunette were driving to a party and they were speeding because they were late.

"Watch out for cops," the brunette said.

They drove on for about five minutes when suddenly the blonde said nervously, "I think that's a cop behind us."

"Is it after us?" the brunette questioned.

"Er, um..." answered the blonde.

"Well, is it?" asked the brunette with a growing temper.

"I don't know..."

"Well are it's lights on?" insisted the brunette.

Replied the blonde, "Yes... no... yes... no... yes... no... yes... no..."