When you learn "I'm Gettin' Nuttin' for Christmas" in elementary school, one question arises: Why the hell are we singing this old-ass song? The second question you'd ask, though, is "What the hell is a penny slug?" Because nobody under retirement age has ever seen one, let alone used one.
Well, despite the advent of digital coins in a digital age, penny slugs and fake coins still exist. And if you've ever wondered how vending machines spot the fugazi, don't fret: this YouTube video from the Science Channel, which is also going nuts on reddit in gif form, explains it all for you.
Much like the Catherine Zeta-Jones-butt-exploiting film, Entrapment, your vending machine uses a system of lasers and electromagnets to figure out how large and round your
butt change is, thus directing the real coins toward the appropriate coin slot (another butt reference, for the uninitiated), while shunting the frauds to the discard chute. The difference here, though, is you end up with a $1 bag of Funyuns instead of a priceless Chinese mask and sexual tension with Sean Connery.