What is the mechanism by which rubbing allows charge to transfer b/t objects?  It seems like such a simple thing that is overlooked in the physics textbooks, but I don't understand it.  I understand this much: charges go to the material with more electron affinity.  The rubbing action maybe produces heat at the surfaces allowing more electrons to liberate freely.  My question though is, is heating all that is involved?  Don't substances tend to discharge when you don't rub them together, which would imply that it's something more than just heating?  would two articles be charged if you merely heated them up and held them together?  If it's not just heat, what else is going on here?  Thanks so much, as I'm reviewing for the GREs its these sort of questions that don't shake themselves loose for me.