I just read this a couple days ago in Mystery Method (which I highly recommend) and I thought I would share. It's pretty funny - the second you read this you begin to notice it happening a lot more.

Proximity
When you are in the field (club, restaurant, mall, whatever) a girl might consciously or subconsciously reveal interest in you through proximity. This is often the case with shy girls. If a girl is revealing interest in you through proximity she will typically be 5-6 ft. away from you and facing a different direction. If you notice a girl in this range of proximity several times over a short period of time then you can open her and she will be receptive. Mystery says that she might actively be doing this in the hopes that you will say something or she may be subconsciously doing this. Either way she will be receptive to an opener.

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You're not gonna be having any less sex with her if you don't talk to her.