Little Johnny was visiting his Uncle's farm in the country when he rushed into the farmhouse and yelled, "Uncle, come quick!
The bull is fucking the brown cow!"
The farmer shook him by the collar. "The vicar is coming this afternoon, so watch your language," said the uncle, "If it happens again, then just say the bull is playing one of his dirty tricks, right!"
Johnny agreed and went back outside. About an hour later the vicar was sipping tea and eating cucumber sandwiches with the farmer and his wife, when Johnny came running in.
"Uncle, the bull has played a dirty trick on the brown cow!"
"Really," asked the vicar innocently, "How?" "Don't worry," replied Johnny, "He's just fucked the white one instead!"