Whenever I take a shower with the bathroom windows open, the curtains billow into the stall and drape themselves clammily and unpleasantly around my torso and nether regions - why is this? My WAG is that the rising hot air in the stall creates an updraft which draws the flimsy curtain in after it, but then my degree’s in English Lit, not physics: any better answers?
10 points for Scissorjack!
Hot air rises in the shower, cool air comes in from near the floor to replace it, but the curtain’s in the way, so it billows inwards.
Subtract 10 points for putting this in the wrong forum: could a kindly mod move it to GQ, please?
The shower curtain will blow in regardless of whether the water is hot or cold. 
See the actual answer from below:
http://www.straightdope.com/columns/010810.html
Essentially it has to do with the water pushing the air in the stall, not the temperature (though I would assume it helps.)
OK, moved to GQ from Comments on Staff Reports. But, as noted, it’s covered by Cecil.
Buying shower curtains/liners with little magnets along the bottom really help minimize this effect. I’m always a little impressed (when I ever bother to stop and think about it) at how strong those tiny magnets are. We have a window that opens into the shower in this apartment, and even when it’s closed, it doesn’t really tightly seal, and it seems to prevent this from happening too.
I notice this every single day when I step into the shower, and although I haven’t read Sage Rat’s links yet (but I will right after church this am), I always attribute it to something I learned in high school physics: High-Speed-Low-Pressure the same effect that causes the lift which keeps airplanes aloft.
The shape of the upper part of the airplane’s wing creates a longer distance for the air to travel (compared to that of the underside of the wing). So, the air must travel faster over the top of the wing.
The shower of water speeds the air past the curtain creating high-speed-low-pressure over the inside of the curtain and the higher pressure outside provides “the” lift which causes the curtain to billow inward.
From what the professor maintains in the first link, I was right about the high speed low pressure, angle, and Wikipedia talks about a horizontal vortex of low pressure, which I count as a “Maybe partly.”
Is there a Nobel Prize in the offing? (If it helps, my name in a previous life was Bernoulli.) 
Although this effect exists and is measureable, it only accounts for a tiny fraction of the lift produced by a wing. Most of the lift is just from the fact that the wing is tilted, and so deflects air downwards. All of the lift can be accounted for through Bernoulli’s equations, but the full Bernoulli equations are a lot more complicated than just that one effect, and in fact are just a special case of F= ma, as applied to fluids.
I was wondering about the matter of the shape of the upper half of the wing, after I submitted that post.
Think back when you were a kid and you threw those very simple balsa-wood planes into the air. They flew beautifully and gracefully - and none of them had that shape I mentioned in my post that would provide the high speed low pressure. Not only that, but many, many of them had wings that were utterly flat. not tilted, but parallel, in fact with the earth, if you lay them flat on the ground.
So I’m very confident you;re right, Chronos.
If this accounted for a good part of the lift the wing provides, how could a plane fly upside-down? People who think that explanation is all there is to it have obviously never been to a good air show. 
As for the balsa planes (and paper airplanes, and dust motes, and individual hairs), remember how light they are, and how well they catch any passing air current. In any atmosphere, every object has a terminal veolcity: The maximum speed it will reach when dropped from an arbitrary height within that atmosphere. That is, no matter how high you go within the atmosphere you will never make it fall faster than its terminal veolcity. That’s because terminal velocity is the balancing point between wind resistance (drag due to the atmosphere) and acceleration due to gravity: Gravity can’t make it go any faster and the air can’t make it go any slower.
The corollary of this is that in the absence of an atmosphere, everything falls at the same velocity. This was demonstrated when some of the astronauts who went to the Moon dropped a feather and a hammer: They did indeed fall at the same speed.
It’s possible to have negative terminal velocity: Eagles riding thermals and a sheet of paper being blown upwards by a hot air register.
Terminal velocity has interesting implications for biology: No matter how far an ant falls, it will never be damaged. Its terminal velocity is too small for the force of impact to harm its body.
From The Biology of B-Movie Monsters (a fascinating essay on how big and small things can really get, compared to their B-movie counterparts):
I forgot a good part of why balsa planes fly so well: Low mass means low inertia, which means they don’t resist direction changes or speed changes. Any vagrant current can move them around at will.
Buying shower curtains/liners with little magnets along the bottom really help minimize this effect. I’m always a little impressed (when I ever bother to stop and think about it) at how strong those tiny magnets are. We have a window that opens into the shower in this apartment, and even when it’s closed, it doesn’t really tightly seal, and it seems to prevent this from happening too.
Wow, that’s an interesting double post! I went out to run some errands, saw a timeout message on my browser and tried again. Sorry!
But the magnets do work well!
From the Wikipedia article:
One solution to the problem is to use shower curtains with magnets strung along the bottom that affix to the side of a metal bathtub. Another solution, first proposed in a CSIRO publication, is to use a door, not a curtain.
I LOL’d. In a Library computer lab. Fortunately, it’s Sunday, and I’m the only person in here. (Hell, I’m only here cause I WORK here.)
After extensive research, I’ve determined that magnets are not effective with fiberglass tubs.
Ultimately, the question remains: would a shower curtain billow inwards if the tub was on a running treadmill?
Why use magnets at all? Wouldn’t simple lead weights in the seam work just as well?
My shower curtain does not do this. I have never lived in a house where the shower curtain did this. Sure, it may have happened to me in a shower in a hotel room once or twice, but I had never heard of this as a widespread problem. I mean, I just thought it was a hotel thing, like little bars of soap.
I feel so alone.
Depends on the curtain – some are just water resistant fabric, but others are actually heavier plastic. (I wish I’d gotten one of those plastic liners, mildew or no mildew, due to the aforementioned billowin’ action)