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DEBBIE ELLIOTT, host:

Science is all around us. Sure, you can live your life oblivious to the laws of physics and the basic principles of biology, but at some point you've probably asked yourself questions like, why is blood red? Or why is the sun yellow? Sometimes scientists have the answers to these seemingly simple questions. Sometimes they're still trying to puzzle them out.

Today we start to try to answer some of those questions in our new series, Science Out of the Box. We'll be considering matters grand and mundane. Today, we begin with the mundane and NPR's Joe Palca in the bathroom, at the shower.

JOE PALCA: To begin with, I want to assure everybody I'm fully clothed. I'm going to turn on the hot water, close the shower curtain and watch what happens.

(Soundbite of water)

PALCA: Okay, it's doing it. The curtain is gently bowing inward. And I should note my shower curtain is this airy little number. It doesn't have any weights at the bottom. So why does the curtain creep inward when you take a shower? To find out, I've got Liz Marshall on the line. She's an engineer with Fluent, Inc. The company has developed software to simulate water flow, and it's been used to model the shower curtain phenomenon. Hi, Liz.

Ms. LIZ MARSHALL (Engineer, Fluent, Inc.): Hi, Joe.

PALCA: And hang on a second because to save water I'm just going to shut off the shower here. So you know, there have been several attempts to explain this shower curtain business. I gather one involves hot water?

Ms. MARSHALL: Yes, that was a popular belief a while ago. People recognized that the hot shower water heated the air inside the shower curtain, and hot air rises. And so the air inside, it's less dense than the air outside the shower curtain. It would rise and pull in the cooler air from outside the shower. But then people also noticed that the shower curtain was pulled in, even in a cold shower situation.

PALCA: And then the other one I've read somewhere is there's - some people say the Bernoulli Effect. What's that about?

Ms. MARSHALL: Right. Well, the Bernoulli Effect says that as the fluid accelerates, as it goes faster, the pressure drops. So the idea behind the Bernoulli Effect in the shower is that as the water droplets come down from the showerhead, they drag the air along with it and that causes the air inside the shower to move. And therefore the pressure inside the shower reduces, and there's a pressure difference between that moving water - or the moving air, I should say, inside the shower curtain, and the still air outside the shower curtain that causes the shower curtain to be sucked in.

PALCA: So the Bernoulli Effect is one possibility, but there are other theories out there, right?

Ms. MARSHALL: Yes. In fact, David Schmidt, who's a professor at the University of Massachusetts, did a simulation about five years ago on the computer where he modeled the air flow in the shower that's generated by the spray coming down.

And what he found was that the motion of the air inside the shower is actually in the form of a spinning vortex. And the spinning vortex is very much like the eye of a hurricane with a very, very low pressure at the center of it. And so it was really that low pressure that he found, which was lower than the pressure anywhere else inside the shower, that was probably the dominant mechanism to cause the shower curtain to be sucked in.

PALCA: So it's the low pressure at the center of the vortex that's drawing the shower curtain toward the back wall of the shower.

Ms. MARSHALL: That's right. And the shower curtain is attached at the rod, so it can't be sucked in there. The only place it's really free to move is down at the bottom, where it's not attached to anything.

PALCA: Do you think we'll ever know for sure? I mean this is just a simulation that he's done.

Ms. MARSHALL: It is. Well, I think...

PALCA: Has he taken this into the lab?

Ms. MARSHALL: Well, I think people could find out doing more sophisticated calculations. They could put a person in there. They could do experiments that vary the temperature and the material of the shower curtain. So yes, I think that people could really get a much better handle on this in the future.

PALCA: I see a Ph.D. thesis in the making. Liz Marshall is an engineer for Fluent, Inc. in New Hampshire. And we'll talk to you again, I hope.

Ms. MARSHALL: I hope so, too. Thanks, Joe.

PALCA: I'm Joe Palca, NPR News, in my bathroom.

ELLIOTT: That's why I always buy the shower curtains with the little weights at the bottom. And there's more Science Out of the Box on our Web site, npr.org. If you head there, you can find the answer to this burning question from a listener: Why do sparks fly when metal meets the microwave?

Coming up on ALL THINGS CONSIDERED, apropos of the bathroom, the new movie Flushed Away.

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