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When I make eye contact with a person and smile at her/him, I do not have any interest in that person other than to see whether I know that person or not.A smile means strictly nothing. It could be because that person's behavior at the time of eye contact reminded you of someone or a situation or his facial expression caused a smile. There are so many different possibilities. It would be a different thing if the girl turns around and assess the guy or makes a comment that would lead to believe that there is some kind of interest.
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