This was a thread a long, long, long time ago on the old DM's forum. The basic idea is to alter the lyrics of a Beatles song so that they have a farm/barnyard theme. Or, find song titles that could fit in the category. It's fun, I promise!
So, here's a little bit that I remember from back then:
"She said she'd always been a heifer
She chewed on 15 bales a day
and though she thought I MOO the answer
Well I Mooed, but I could not neigh"