"The Circle Flies" joke

An old farmer was hauling a load of manure when he was stopped by a state trooper. "You were speeding," the cop said. "I'm going to have to give you a ticket."

"Yep," the farmer said as he watched the trooper shoo away several flies.

"These flies sure are terrible," the trooper complained.

"Yep," the farmer said. "Them are circle flies."

"What's a circle fly?"

"Them flies that circle a horse's tail," answered the farmer. "Them are circle flies."

"You wouldn't be calling me a horse's ass, would you?" the trooper angrily asked.

"Nope, I didn't," the farmer replied. "But you just can't fool them flies."