A Horse with Nine Legs

What is wrong with the following argument? Do you know the name of the error being made?

"One horse has four more legs than no horse. No horse has five legs. Therefore, one horse has nine legs."

Digging Up the Past

A friend of yours returns from an archaelogical dig in Italy and shows you a coin she dug up. the coin looks very old and is inscribed with thte words "47 B.C." Your friend offers to sell it to you for just $20. Do you take her up on the offer?

Outrunning the Tiger

Two explorers are walking in the jungles of India. They come to a clearing, where they see a tiger. The tiger snarls and starts to approach them. Quickly, one of the exploreres diges into his pack, pulls out running shoes, and starts putting them on. the other explorer looks at him quizzically and says, "Surely you don't think you can outrun a tiger?"

"I don't think I can outrun the tiger," replies the explorer, "but these running shoes will still save my life."

How will the running shoes save him?

An Empty Riddle

The more you take away from me, the bigger I become. What am I?