But as for that mark, I'm not sure about it; I don't believe it was made by a nail after all; it's too big, too round, for that. I might get up, but if I got up and looked at it, ten to one I shouldn't be able to say for certain; because once a thing's done, no one ever knows how it happened. Oh! dear me, the mystery of life; The inaccuracy of thought! The ignorance of humanity!