The one my husband and other family members still find funny. The difference between a bull and a steer. I thought bulls had horns and steers didn't (My Uncles polled their steers which I had never been near enough to know).
Man you can tell I come from the city with this one.
Oh yeah, my son thought chicken's laid eggs in the cartons and that they were always clean. I thought he'd never eat another egg when he found out differently.