Girls, Why do girls touch their hair after looking at me?

If they smile while touching their hair obviously I will take that as a sign of interest and approach her but often times , when i use my peripherals to see , a girl looks at me for a few seconds then touches her hair. Sometimes after making eye contact. I want to know if its out of nervousness or if she is flirting i don't know.
Girls, Why do girls touch their hair after looking at me?
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