So the farmer took the son aside and told him to go watch the bull and let him know when he serviced the white cow.

So the farmer took the son aside and told him to go watch the
bull and let him know when he serviced the white cow. (Only he
did not say serviced.) "But son when you come back tell me the
bull 'surprised' the white cow, and I'll know what you mean."
A little while later the son runs up excitedly and the farmer
is prepared to avert another disaster. He says "Don't tell me
son, the bull 'surprised' the white cow."
"He sure did, he serviced the brown cow again!" (Only he did
not say serviced.)