Pat the dog

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 stroke 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!'