Dude, welcome to 1998. I knew about this back when I was bagging groceries... When I was 15!!

Considering the heads of most scanners are spring loaded to ensure good contact and that the thickness of a grocery bag is about .8 mils, it hardly seems likely that the "added thickness" is what's at work here.

I've watched 15 year old baggers execute this with total confidence, but I'm still baffled by the mechanisms at work.

Anyone have a more plausible idea?

Ideas are always accepted... Even though this technique is known for some veterans back from 1998, I appreciate the one who posted it for others who don't know. Keep posting....

"Vidhya Dadati Vinayam..."

It's nice and all but this dosen't explain how it really works.

It's nice and all but this dosen't explain how it really works.

I have also known about this for years, but I assumed it had something to do with the tendency of plastic bags to build a static charge which amplifies the polarization of a worn out magnetic strip making it easier for the reader to pick up.

i was a cashier at a local grocery store, and i used this trick often.

my explanation.

when the bag is opened it creates a charge... insert card. the electric from the bag charges the magnetic bar temporarily creating a electro magnet. magnetic field from bar passes though charged bag and bang it works.... lets just be happy that our "cards" do not use rare earth magnets

If your credit card has this problem a lot, you should just place a piece of tape over it. It is the added thickness of the bag which causes the "magic."

Or you can call the 1-800 number on the back and ask for a new card!

This trick works because the magnetic strip gets worn down over time and might have small scratches in it and the plastic bag or piece of tape basically makes the strip "whole" again by putting a protective layer over it