If I make eye contact with a girl and she immediately looks away, what does that mean?

Basically, when I'm in public and walking from place to place, I'd see attractive girls and as I pass them by, I'd give a light smile and then we'd make eye contact for a split second before she immediately looks away and continues walking. Similarly, while sitting down at public places such as restaurants and making eye contact with a girl for a split second before she looks away.