So, there you are, scrubbing away, belting out your favorite tunes, when suddenly… you’re attacked! By a flimsy piece of waterproof fabric. We’ve all been there at some point, grumbling as we try to un-stick ourselves from the shower curtain that’s randomly decided to billow in at us – which kind of raises the question: why does this always happen?

For a seemingly everyday problem, finding a solution was much harder than you might think – and the true answer is probably not the explanation you’ve been given before. One of the most popular answers, for example, is that of Bernoulli’s Principle – a theorem from fluid dynamics which links the speed of a fluid with its pressure and height. 

“The Bernoulli effect is the principle that explains how an airplane’s wings produce lift,” explained mechanical engineer David Schmidt in a 2001 article for Scientific American. “It says that as a fluid accelerates, the pressure drops.”

Basically, the idea is that water from the shower speeds up the air around it, decreasing the pressure on the inside of the curtain. Meanwhile, no such effect is going on outside the shower, and so the higher pressure forces the curtain inwards.

But there’s one rather large problem with this explanation: “the Bernoulli effect is based on a balance between pressure forces and acceleration, and does not allow for the presence of droplets,” Schmidt explained. “Nor, according to my calculations, is it responsible for the curtain deflection.”

So much for Bernoulli – perhaps the other popular explanation can help us out?  Known as the “buoyancy theory”, this one says that the hot water from the shower causes the air inside the curtain to heat up, thus reducing its density. Again, the air outside the shower is unaffected, and so this cooler, denser air rushes inwards, pushing the curtain in as it does so.

But again, we see a flaw with this argument: “the curtain will suck inward toward a cold shower, too,” Schmidt pointed out. Throw that theory in the trashcan too, then.

Luckily, Schmidt had at his disposal not only a weird fascination with disobedient shower accessories, but also precisely the knowledge and equipment needed to figure out the real answer. As a professor of Mechanical and Industrial Engineering at the University of Massachusetts Amherst, specializing in the dynamics of liquid sprays, his day job involved computer simulation technology that, well, pretty much nobody else in the world had access to.

“I realized that [others] were all weighing in with their opinions,” Schmidt told Wired in 2001, “and with these computer simulations I was doing, I had something at my fingertips that I could use to answer it.”

Unlike previous explanations for the shower curtain effect, Schmidt’s program allowed him to include thousands of minuscule real-life nuances – the distortion of the shower droplets as they fall through the air and interact with each other, for example; the effects of the droplets breaking up as they fall, and the impact of drag on their speed and direction; all things that big-picture answers such as the Bernoulli Principle or the buoyancy effect simply couldn’t account for.

It took two weeks of calculations – this was 2001, after all; computers were slower back then – but Schmidt’s work came good. After years of debate, the shower curtain problem finally had an answer – and, for his discovery (which he swears wasn’t done on company time), Schmidt was awarded the Ig Nobel Prize in Physics.

“Basically, a vortex sets up,” Schmidt explained. “It’s like a hurricane (of air) turned on its side, and in the center of that is low pressure, and that pulls in near the middle of the curtain. But because of the way tension works in a curtain you get the bottom moving in.”

Luckily, the effect is pretty weak – so if you’re really bothered by the daily attack of the shower curtain, there’s an easy solution to that, too.

“The forces generated by this airflow […] are only sufficient to pull light, thin curtains inward. That explains why people with heavy plastic curtains typically don’t have this problem,” Schmidt wrote in Scientific American. “Also, if someone has poor water pressure or a poorly atomizing showerhead, they may not see the curtain suck in.”

So, he advised, “the easiest thing to do is to sew weights in the bottom. Or, if you have a metal tub, magnets can hold the curtain in place.”