A blonde and a brunette are out driving, and the brunette tells the blonde to look out for cops - especially cops with their lights on. After they've been driving for a while, the brunette asks the blonde if she's seen any cops.
"Yes," says the blonde.
"Are their lights on?"
The blonde has to think for a moment, then says, "Yes. No. Yes. No. Yes. No."
"Are their lights on?"
The blonde has to think for a moment, then says, "Yes. No. Yes. No. Yes. No."