ok, here goes. Playing with someone's hair is something you will probably enjoy, too. It's not like it's a chore. Most guys seem to love doing it.

first of all, don't ruffle vigorously. she's not a dog. Don't mess up her hair if she's done it up elaborately, and understand that sometimes she may have a bad hair day and not want you to go there.

If she dyes her hair a lot and/or complains of brittle/dry hair, then don't do anything like twist or bend her hair too much.

Other than that, there's not a whole lot you can do wrong. Have you ever observed girls playing with their own hair? I guess it's somewhat like that. Run your fingers through it. Curl a strand around your finger. I actually love doing this with my own hair because it's kinda wavy towards the ends, and I flatten one strand out and watch how it re-waves itself. Re-arrange her fringe. Pretty much just have fun with it.
Playing with hair can be intense and flirty, or it can be akin to absent mindedly toying with a rubiks cube or other such gadget. There's fitting situations for all kinds of hair playing

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