When a beautiful girl walks into your field of awareness, you have a tendency to notice them.
Your initial glance will be longer than usual to ensure that the first moment's intuition was correct. Their presence now occupies a fraction of your focus, regardless of your consent. They are interesting.
You unconsciously memorize their approximate position in the room. You sneak peeks at them. You can't help yourself from focusing on them at least once every time your eyes dart around. You utilize a variety of observational tricks to obtain more data about her while avoiding the observation of your observances.
What you mayn't know is that she knows. She knows that wherever she goes, people will be watching her. She may even assume everyone is gazing longingly. When she catches your eye, she knows that wasn't the first nor the last stare.
You have now sparked her interest. She will wonder why you were looking at her (little wonder) and experience a slight increase in self-awareness. Since she is curious, she will steal glances to see if you are looking again. She wants to catch you.
If you get caught too many agains, she will feel uncomfortable. You have broken social protocol, and infringed upon her privacy. If you have a functional understanding of social norms, you will feel embarrassed and stop.
This effect isn't limited to beautiful girls, but is most beautifully illustrated as such. You need only be extremely interesting relative to the others around to receive such attention.
Having lived for three months in a place where my physical appearance made me one in a million, I understand the point of view of the starkly interesting. Being a 21 year old male, I know how it is to be helplessly interested.
-dougH