Nothing that important or exciting. Just me, raw, vulnerable, open, me.
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The tears on young Sherlock’s face are probably thought to be just a child’s reaction to an intimidating situation. But everything else about the adult Sherlock is reflected in the young one, so why not the tears as well? Showing us young Sherlock allows us to see what Mycroft sees: a reflection of his baby brother as a scared little child. What we don’t see, what we don’t realise, is that it is not the child Sherlock crying. It’s a two-fold deceit: Sherlock turns his back to John and only then the camera goes to a close-up to show the absolute heartbreak on his face. But in the next instance they switch to young Sherlock whose natural reaction is to start crying and therefore the audience doesn’t think anything of it. So neither John nor the audience sees the adult Sherlock, the real Sherlock, in tears.