A farmer got pulled over by a state trooper for speeding, and the trooper started to lecture the farmer about his speed, and in general began to throw his weight around to try to make the farmer uncomfortable. 

Finally, the trooper got around to writing out the ticket, and as he was doing that he kept swatting at some flies that were buzzing around his head. 

The farmer said, "Having some problems with circle flies there, are ya?" The trooper stopped writing the ticket and said-"Well yeah, if that's what they are-I never heard of circle flies." 

So the farmer says-"Well, circle flies are common on farms. See, they're called circle flies because they're almost always found circling around the back end of a horse." 

The trooper says, "Oh," and goes back to writing the ticket. Then after a minute he stops and says, "Hey... wait a minute, are you trying to call me a horses ass?" 

The farmer says, "Oh no, officer. I have too much respect for law enforcement and police officers to even think about calling you a horses ass." 

The trooper says, "Well, that's a good thing," and goes back to writing the ticket. 

After a long pause, the farmer says, "Hard to fool them flies though."