Farmer borrows his neighbor's bull for stud. He leads the bull to a pasture where his white cow and black cow are waiting. He leaves his 10-year-old boy in charge. "If anything happens, son, you just come runnin' home and tell me."
Farmer returns home, finds the circuit ridin' preacher seated at his table, discussing religion with his wife. He sits down, looks solemn, takes a cup of coffee, when the kid bursts in:

"Hey Dad, guess what? The bull just f**ked the white cow!"
Wife turns crimson, preacher nearly faints. Farmer grabs the kid by the ear and takes him outside. "Now lissen: watch yer language! Go back out to the pasture. If anythin' happens with the black cow, you come back and tell me, 'Dad, the bull just surprised the black cow.' Got it: 'The bull surprised the black cow!' "

The farmer returns to the religious discourse with the circuit-ridin' preacher and his wife. About 20 minutes later, his son comes bursting in:

"Hey Dad, guess what?" the kid yells.
Farmer leaps up: "I know: the bull surprised the black cow!"

"I'll say he did," the kid beams. "He f**ked the white cow again!"