As a piece of food approaches your mouth, you can generally preconceive the degree of softness and adjusts how hard you're going to bite down to penetrate it.
Whether it's a marshmallow or a piece of chewy beef, however, a two and a half year old apparently bites down as hard as he can to get his teeth through as fast as possible. If dad's finger happens to be in there a, it is painful as hell.
Of course, since biting is not allowed, Kiernan was inconsolable - despite being told it was an accident and telling him the throbbing digit was okay (lie to your children, everyone!) he sobbed for at least ten minutes. You'd think I bit him.