A farmer had 5 female pigs.
Times were hard, so he decided to take them to the county fair and sell them.
At the fair he met another farmer who owned 5 male pigs.
After talking a bit, they decided to mate the pigs and split everything 50/50.
The farmers lived 60 miles apart, so they agreed to drive 30 miles each and find a field in which to let the pigs mate.
The first morning, the farmer with the female pigs got up at 5am, loaded the pigs into the family station wagon, which was the only vehicle he had, and drove the 30 miles.
While the pigs were mating, he asked the other farmer, "How will I know if they are pregnant?"
The other farmer replied, "If they're lying in the grass in the morning, they're pregnant. If they're in the mud, they're not."
The next morning the pigs were rolling in the mud, so he hosed them off, loaded them into the family station wagon again and proceeded to try again.
This continued each morning for more than a week.
The next morning he was too tired to get out of bed.
He called to his wife, "Honey, please look outside and tell me whether the pigs are in the mud or in the grass."
"Neither," yelled his wife, "they're in the station wagon and one of them is honking the horn."