Paddy and Murphy buy a horse each, and they are watching them in the field trying to figure out how to tell them apart.

After a while Paddy says "I know - we'll paint the top of one horse red, and the bottom of the other blue."

Murphy says "That's no good, the rain will wash it off."

After a while Murphy pipes up "I know - we'll shave the front of one horse, and shave the back of the other."

Paddy says "That's no good, it'll grow back."

After thinking for a while, Paddy shouts "I know what the difference is - the black one's bigger than the white one."