In the last month I have been asked over 10 times if Knightley and Mason are twins.  Usually this occurs when there are standing a few feet apart or sitting down so you can not see their height difference.  At first this question surprised me, I didn't think they looked that similar but  after hearing it over and over I have noticed how similar they do look.  And sometimes now I am blown away by the fact that, even to me, in the right light or right outfits, they do look identical.