Tar?

Perhaps I yawned and missed something but I didnt understand the whole tar scene. How did it get in her hair? From laying on the roof and she inexplicably hadnt noticed before she went to bed? Had the grandmother snuck in and put it in her hair and inexplicably not woken her up? The grandmother demands Chris cut her hair for being insolent. He refuses. She withholds food for days then finally gives in. And THEN she gets tar in her hair and he cuts it? And the grandmother never says word one about it after, so theres no acknowledgement that she got her way IF it was her doing or even any snide satisfaction about it at all. It just disappears as a plot point from the movie entirely at that point and Shipka just continues the movie as short haired Cathy and everyone is fine with that. The whole thing was perplexing and random to me. Why was this scene even there? Was this the directors awful attempt to make Cathy look different from her mother so the audience could understand the growing difference between them (not a reflection of her mother). If so then he would have been better off having her cut her own hair out of anger or bitterness or something which, although cliched (White Oleander anyone?), would get the point across. And not just have this random scene where theres no reason for the hair cutting and nothing comes of it anyway.

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Using words to describe art is like using a screw driver to cut roast beef.