What does a smile mean?

Smiling is all part of that body language and subcommunication stuff, but what exactly could a girl smiling at you mean?

Whats going on in their head when they smile at you? Like, today 2 girls that I've never spoken to before smiled at me, one when I took a paper from her and the other when i gave a paper to her...

I realized that a smile is non-verbal, so it counts as an action, not spoken word.

Also the girl seemed to look down when she smiled. Is this trying to say something like, you're hot but I want you to talk to me before I talk to you?
Or it could just be like saying hi, which happened with a differen't girl today, but I actually know her so there isn't attraction.

Bottom line question - What is a girl thinking and feeling when she smiles?

also what does it mean when they rub their foot up your pant leg under the lunch table

it means that your such a lonely fag you read into things too much