Why my student stares at me?

I'm a college professor and I have a student in my class that always stares at me. I'm used to students looking at me during my lecture. But this woman is different. Sometimes I'm checking my email in the computer and once I turn, there she is staring at me with a smile on her face. The weird part is that once I look at her she looks away. The thing is that she always looks at me when I'm not looking. Everytime I've caught her she's smiling and looking at me and turns her look somewhere else really fast. Then I offer my help but she replies she doesn't need my help. Why she behaves like this?
Why my student stares at me?
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