The farmer tells his son to go out in the back 40 and watch
the bull. Then come and tell me when he services the white
cow. (Only he is a back wood farmer and does not say service).
The farmer goes back to the house to find the preacher and his
wife have dropped in unexpectedly. He is having tea with them
(if you can believe that) when the son comes home and tells his
father that the bull serviced the Brown cow. (Only he did not
say serviced.)
The preachers wife heard the son's language and asked "What did
he say?" To cover up the farmer said "He said the brown cow
bucked." (Which of course rimes with what he really said.)