One day a man is walking down the street when he sees an old man with a nice looking dog. He goes over to the man and asks, "Does your dog bite?"

The old man replies, "No, never."

When the man bends down to pet the dog, it immediately takes a snap at his hand. The man says, 'I thought you said your dog did not bite!"

"I did," replies the old man, "but this isn't my dog!"