I often see that when you lock eyes with a girl you should smile (then if she responds you approach).

Now it may sound stupid but Im used to being stoic...

HOW DO YOU SMILE? Teeth? No teeth? Smirk? Grin?

My logic would be do it naturally but I find it I look more like a friendly kid when I do so.

Also do you smile within a second? After 2-3 seconds seems too late.