Why do cow milking stools only have three legs?
Because the cow has the udder!
Okay, listen up, Ted. You're probably the kind of guy who wears socks with sandals. This joke is about why a milking stool has three legs instead of four. You see, the cow's udder takes up one of the spots where a leg would be so, like, duh, three legs, ya dig? It's called a pun.