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I though that it could be that, but then I thought, growing taller and growing legs are entirely different things, it can't be that, so I decided to google for sayings and idioms. :D

But if it was possible to get some of someone elses legs, it would be awesome to get their knees! So I would have double knees in one leg. I could bend to do anything. Of course the kneeless person then would have to grow new ones. That would suck. But not for me because I would be all bendable!