Why do girls do this?

I know I previously asked this but it got me thinking. I had said that I am an attractive 6'2 guy. I have always been wondering why girls make eye contact with me. LIKE DIRECT EYE CONTACT THEN LOOK AWAY. Are they checking me out or they looking at my eyes... i do have nice eyes... kinda dark green with a dark blue around the edges... but I don't think they would notice that from a distance. Is it in the nature of a girl to check out a guy or what are they doing and why is it so direct.