This is the stock response, and it simply isn't true. The capacity to injure is on a completely different level with these breeds, which is why the government tried (and failed) to ban them years ago. Any dog can turn if provoked, sometimes for reasons that wouldn't occur to its owner. The difference is that a collie, greyhound, spaniel etc would inflict a bruise and then run away. Pitbulls and staffies are far more likely to attack in the first place; that's what they were bred for, there is no other reason for their existence. And once they do attack, their jaw mechanism clamps shut on its prey. We were out on a delivery once in a particularly shit area, and a bloke walking his chihauha on a lead had it attacked by a staffie which shot out onto the pavement. Five of us (including the staffies owner) couldn't get it to let go until someone suggested getting a crowbar between its jaws, it literally took that to get it off. The small dog was still alive (just), but torn to pieces.