It’s one of life’s great mysteries. You’re in the shower, shampooing your hair and thinking about your day, when you suddenly feel something brush against your leg. You quickly turn around to discover that it’s only the shower curtain, billowing in towards you. But why is it doing that? The room is closed and there aren’t any fans. What could be blowing the plastic into your body?

Interestingly enough, it’s only recently that a probable answer has come up. David Schmidt of the University of Massachusetts ran modified software from Fluent Inc. on his personal computer that was designed to simulate how sprays interact with their surroundings. After a lot of math and 2 weeks of using the program, he had a model of 30 seconds of shower time.

What did this reveal? That the spray created a horizontal vortex, running perpendicular to the shower curtain. Inside the vortex a low-pressure region formed, which in turn is what pulls the curtain in towards the person. Luckily, these forces are pretty minimal, so attaching weights, or even buying a thicker curtain, are more than sufficient to put a stop to it, but it’s interesting to think that you are creating a tiny weather system in your bathroom every morning.

Click here to learn more about the experiment from David himself in Scientific American.