The other day I ran across a writeup for a general physics experiment (it might have come out of a high school AP course - I'm not sure) about using charged Scotch tape to separate charges for the purpose of measuring the charge through a torque equilibrium setup. The technique was to put a piece of tape, sticky side down, on a surface, put another piece, sticky side down, on top of the first, and then rip off the top piece and use it to make a crude electroscope. So, I understand how this works (it requires a fair amount of hand-waving and beard-muttering to say this is a good example of Coulomb's Law, but whatever...) but I came up short as to why the writer thought it plainly obvious that the top piece would be negatively charged. I thought it might just be a slip till I found another similar writeup from a University I won't name stating pretty much the same thing. What am I missing? What is there about Scotch tape that makes it so clear (notice I avoided any pun about transparent) that the top piece would be negative? Besides, I can't get one to be negative.