What does it mean when a girl bites her lip at you?

Anonymous
This happened to me twice recently. The first was a girl at work who seemed to do it absentmindedly while I was talking. The second was when I was talking to a girl with another guy present. She seemed to laugh more at his comments and they know each other better so I assume she's more into him.

But she asked me questions (we were studying for a test) and actively brought me into the conversation. She bit her lip while I was talking once or twice and unlike the other girl was smiling at the same time.

Why do girls bite their lips and can you explain (or at least speculate) these two examples? I haven't noticed it before but this happened twice in one day and I don't know what to make of it.
What does it mean when a girl bites her lip at you?
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