system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 : How a vending machine knows when you insert counterfeit money?
gpt
Wrong size, wrong weight, doesn't respond the way it should to an electric current, etc. Coins and notes/bills all have exacting technical specifications, and if something inserted doesn't meet them, the machine is often going to be better at determining it's a fake than the human eye.