An old rancher is stopped for speeding by the highway patrol and is standing at the back of his truck watching the trooper write out the ticket.  A fly is circling the trooper’s head, and the trooper keeps swatting at the fly to no effect.

Circle flies are bad this year, the rancher says.

The trooper continues to write for a moment and then stops, looks at the rancher and askes, what’s a circle fly?

Well, says the rancher, that’s the kind of fly that circles around the asshole of a steer.

Oh, says the trooper, and goes back to writing the ticket.  After a few moments, he stops writing and looks at the rancher and says, did you just call me an asshole?

Oh, no, says the rancher, I’d never say that to a law enforcement officer.

Good, says the trooper, and goes back to writing the ticket.

But, you know, says the rancher, it sure is hard to fool a circle fly.