Like on the arm or whatever. Nothing sexual. If its a girl you have know for couple months or so? What if you tend to always be talking to your close friend and she wants to attract your attention without directly interrupting it? She has good manners... would a light touch to your arm grab your attention? Or is that off limits? Would you shake her off or ignore it?
Also, what are you thinking as she does it?
Will it make you nervous? If you didn't like her would you jerk away?