Pay attention. Is the bar code printed on the back cover of the book concave and convex now? It's just a magnetic stripe inside. ...
The same is true for many goods in the supermarket, except that the bar codes in the supermarket are not magnetized too much. When scanning at the checkout counter, they have been demagnetized ... but there are many things in the supermarket that can't be magnetized ... I won't say the details ... Hehe ~ ~
As for what you said, I'm not sure.
The school librarian's notebook is even more obvious ... Staples. ..