I just remembered that one.
Mine is probably
"Billy what is the plural of ox."
"oxen, I saw a herd of oxen"
"Brian?"
"what?"
"what is the plural of box?"
"boxen, I got two boxen of donuts"
"no, Billy what is the plural of goose"
"Geese, I saw a flock of geese"
"Brian?"
"Whahut?"
"What is the plural of Moose?"
"MOOSEN, I SAW A FLOCK OF MOOSEN"