Why do girls do this? and girls? why would you do this?

A girl in my once a week college class stares at me but when I make eye contact with her, she would hold the contact for a sec or two then look away(this happened throughout the two hour long class). Also, I noticed her feet were always pointed towards me even when her torso was facing somewhere else. So, I took the initiative to say hi to her, she just forced a smile but didn't reply. I figured it's either she's into me or terrified of me. What do you guys think? I'm confused.
Why do girls do this? and girls? why would you do this?
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