During a traffic stop a police officer is swatting at a fly that is circling around his head, and blurts out "what kind of damn fly is that anyhow".
The traffic offender replies, "that's a circle fly".
The officer replies that he's never heard of a "circle fly". The offender replies circle flies are usually found circling around a horses ass.
Enraged, the police officer says, "are you calling me a horses ass?",
To which the traffic offender replied, "no sir, but you can't fool a circle fly".