How Many Legs Does A Cow Have?

There is a story about Abraham Lincoln who was arguing with a political opponent.
“How many legs does a cow have?” he asked his adversary.
“Four, of course,” came the disgusted reply.
“That’s right,” agreed Lincoln. “Now suppose you call the cow’s tail a leg; how many legs would the cow have?
“Why, five, of course,” was the confidant reply.
“Now, that’s where you’re wrong,” said Lincoln. “Calling a cow’s tail a leg doesn’t make it a leg.”