This analogy gives a vivid mental picture when it comes to how you should touch a woman. If you take a marble, put it in boiling water, then drop it into a glass of ice water, it will shatter. The transition is too sudden. However, if you take it out of the boiling water, let it cool, then put it in the ice water it will be just fine. Because there was a smooth transition. And when you begin touching a woman, there needs to be smooth progression, not a sudden jolt.
See, too many guys try to go from having a great time in conversation to "laying one on her" and catching her by surprise.
Many times this will scare a woman who otherwise would have been very happy to kiss you, had you smoothly led up to it. It's the sudden jolt that can scare her off, not the kiss itself. Women like to be kissed AFTER they've been warmed up. After you have relaxed her with your conversation, being funny, naughty, a little cocky, getting her to open up and talk herself.
After she is relaxed you will see a change in her attitude, make her laugh a few times and then touch her in a very non-threatening way. In this first contact you need to show her that you want to be more than a friend, but also that you are not too eager and not creepy and after something. If you get a good laugh brush your hand across hers, this contact could be the beginning of her attraction for you.
The moment feels complete and she will probably think about it a few times. After she responds to your first touch and is still obviously interested in you go for a second touch to her shoulder in passing, if she responds positively to this then you can get a bit bolder. It is obvious when a woman is not attracted to the guy she is talking to, she will not let him touch her and if he tries she will not respond happily.
