Playing with her hair?

How much truth is there in the idea that if a girl plays with her hair when she talks to a guy that she likes him?

Not the last time, but the time before, I saw a friend who I like and when we were talking at 1 point I noticed she wouldn't stop playing with her hair. She knows I like her as more than a friend but when we discussed it once before she said it wasn't mutual.

I know she could have just been fidgeting but I'm just curious as to how true the theory behind it is
Playing with her hair?
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