So I was doing an experiment in my class where my students discovered air pressure. You take two syringes (needle-less, of course), fill one with air, then connect them with a piece of plastic tubing. When you push on the plunger of the syringe filled with air, there's a bit of give, then the other plunger pops up. Voila! Air pressure! This experiment has all sorts of other uses too: fill up the syringes with water instead of air, and you discover quickly that water isn't compressible. It's the reason why we fill brake lines with fluid instead of air, and why air in your brake line results in your brakes not working anymore.
Oh how was I supposed to know what my students would do with this?
Someone in class figured out that, if you take one of the plungers out, you create a mini-vacuum chamber. They experimented with picking up papers, books, and eventually discovered that creating a vacuum on your arm leaves a perfectly circular red spot (see: hickey) on your skin. Next thing I know, several of my students have given themselves perfectly circular hickeys on their arms, necks, and even one particularly red on on the center of a forehead.
So that's why I'm sitting in a bar right now, drinking. Because I need to write a letter to my parents about how their child got a hickey in science class. And because days like this require a drink. There ought to be a law saying that.