"Head-Eater" Cat

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I've notice there's something cute, funny but scary habit my cat Missy has. Every time she notices that me and my boyfriend recline our heads on the back rest of the sofa, she would go on / above the backrest. There she would smell either my head or his head, then she'll bit it! She'll bit our hair / scalp, but the bite doesn't hurt though. When we get our heads off the back rest, she'll stop biting and will lie down or sit on the backrest, as if she's guarding it. Every time she does it, she reminds me of Plants vs. Zombie, that's why me and my boy always joke that we already have no brains 'coz my cat had eaten them off our skulls. It's cute but scary. Does your cat do the same too? Why do you think my cat does that? What do you think about this "cute but scary" situation?