Sam : "Joe, did you know that a cat has three tails?"
Joe: "Don't be silly."
Sam : "But I can prove it."
Joe: "Try it."
Sam: "Well, you'll have to agree that no cat has two tails."
Joe: "Right."
Sam : "And one cat has one more tail than no cat, hasn't it?"
Joe: "Of course."
Sam : "So, one cat has three tails."