Ew Gross

So the other day I was in the Barnes and Noble bathroom, and this woman comes out of the stall with a book from the store in her hand. Hoping that she was just using it as some leisurely bathroom reading before purchasing it, I followed her out into the store. She left the book on a table and walked off! I couldn't believe she did this, and I went back to see if they have some sort of sign up, banning the admittance of unpurchased merchandise in the bathroom. sure enough, they did. Confused, and feeling like I was in an episode of Seinfield, I called my little sister (who used to work at Borders) to ask her what I should do. She told me to stop wasting her time. Even though I'm usually too cheap to buy my books from Barnes and Noble, I was appalled at the thought that this woman had contaminated a book that I might have bought.
As I stood there, dumbfounded and trying to figure out what to do, the woman traipsed out of the store, totally carefree, as though she had not just broken a serious book store law. I witnessed her repulsive act and did nothing. I almost felt like an accessory to her crime. What should I have done? What would you have done in the situation?