What is "chemistry" between two people?

Anonymous
I saw (through a site called formspring) that this guy looked for "chemistry" when he was asked what he looked for in a girl etc.

I know what he means by it yet I have no idea how to describe it...

And I believe I don't fall for a person straight away. Does this mean "chemistry" is supposedly found straight from the first meeting? I am also quiet making it harder for someone to get to know me at first.

So what is "chemistry" as such and how does it... arise?
Updates
+1 y
Also, Do you think its possible for one person to feel chemistry but the other not to?
What is "chemistry" between two people?
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