In the X2 movie, and also in the X-Men cartoon, there is a young woman who is always falling through floors, walls, etc.
How come she doesn’t fall through her clothes as well?
That’s Kitty Pryde aka “Shadowcat” and I don’t know. There was an episode of Star Trek: The Next Generation where Geordi and Ro were “out of phase” with everyone else and could also pass through walls and equipment, but not the decks.
Shadowcat can phase other objects as well as herself. If she holds your hand, she can take you through a wall as well. So I assume she makes sure her clothes are phased as well.
The real question is how come she doesn’t sink into the floor.
I always wondered how she could breathe air that was “in phase” when she was “out of phase.”
Short answer: she can’t. In the comics, she has to de-phase to breathe, at least occasionally - and being phased, rather than solid, is her natural state. It’s tough to get into that much detail in a movie cameo.
You’re right in that she has to de-phase to breathe. But I think the phased state was only her natural state for a short time in the comics. I don’t remember the circumstances surrounding it, but (unless it has changed recently) at the moment her natural state is solid.
And yes, her clothes stay with her because she “phases” them. As for being able to stay on floors… it has been shown in the comics that she can walk on air, so it is sort of assumed that she can stay on floors with ease.
Shadowcat was injured during a fight with with the Mauraders (?), a group that was slaughtering Morlocks. One of them, Harpoon, threw an energy spear at Rogue. Shadowcat grabbed her and phased both of them, but the spear passing through her caused her to stay phased. Of course, she must have been able to breathe, because she was stuck that way for weeks, if not months.
Eventually, her molecules started to drift apart. The X-Men got the help of Reed Richards and the Fantastic Four. There was a whole plot with Dr. Doom, but everything turned out fine.
Out of all the film, you chose that to pick on?
I mean, there’s quite a tradition of superpowers extending to held/worn things. IRL perhaps ‘turning yourself and everything you’re touching intangible’ is a much less likely superpower than ‘turning yourself intangible’, but I simply don’t think X-men is the kind of film where you expect that kind of consistency.
There were some problems I did have, but that wasn’t one of them.
On preview: pehaps I midread your post. If you meant not ‘It would have made more sense if…’ but ‘It would have been a better film if…’ then my post missed the point… 
I think, in the cartoon at least, they make it clear that she’s able to fly, at least while she’s intangible. She’ll rise and fall through the ground with ease. I think there was only one episode where she started falling and couldn’t stop, but that was in the context of a nightmare.
It was explained to me that she causes her molecules to maneuver around the molecules of other objects, rather than going into some sort of alternate dimension or whatever. Presumably, she gets to decide which clumps of molecules to work with… phase through the wall, not through the ground, that sort of thing.
This would explain how she can rise up from the ground: she’s just “climbing” her molecules up around the ground molecules… kind of like a billion tiny little ladders. I don’t think it would work for air, since air isn’t strong enough to support her weight.
“Unstable molecules” - these are an invention of Reed Richards of the Fantastic Four which are used to make superhero costumes. They supposedly have the ability to survive a wide range of superpowers which would destroy clothes made of ordinary stable molecules. They also can duplicate the superpowers of the person wearing the clothes.
Imagine if some fiendish villian (or villianous fiend) secretly hacked into the FF computerized shipping system and rerouted the unstable molecule deliveries. What if the fabric intended for Sue Storm was actually shipped to the X-Men’s Storm. The first time she tried to use her costume it would turn invisible.
Oh, yeah, I hear about people turning themselves intangible in real life all the time.
How about slime molds? IIRC some are practically a slug, but can break up into individual cells, which can later crawl together to reform the organism!. I am not making this up. Let me say that again. I swear on my mother’s grave I saw this on a nature program. I am not making this up.
OK, they only break up into cells not molecules but that’s pretty intangible. And if you put a piece of cloth over one it won’t dissociate the cloth cells and put them back together when it reforms.
On preview: Aha! A cite: http://www.smithsonianmag.si.edu/smithsonian/issues01/mar01/phenom_mar01.html
Um, maybe for the comic book version. But in the movie, she was what, seven years old? I don’t have any particular interest in watching seven year olds slip out of their clothes, and I hope The Controvert doesn’t either.
Now, on the other hand, Storm’s costume turning invisible, now that has some potential…
So she’s able to control her phase while sleeping as well? IIRC she was sleeping when the bad guys broke into X-Men Central and escaped by phasing down through her bed to the floor below. What happens when she gets knocked out? Does she stay out of phase until she’s concious? That would be a great natural defense mechanism as any baddies would be unable to grab her while she’s out cold.
Nope, definitely in her teens (her first appearance was in the first movie, phasing through the door of the study/classroom when Wolvie was in there). Either that or a painfully overdeveloped 7 year old. O_O FTR, the actress playing her (Katey Stuart) in X2 is 18 years old. Sumela Kay, who played her in her cameo in the original is 17 now, so was 14 at the time.
Well, she does open her eyes, draw in a breath (no breathing while phased?) and then drop down through the bed to the next floor.
Assuming you’re not supremely hung-over or disabled/restrained in some way, do you have to take time to work out how to hit the snooze button in the morning, or do you just reach over and tag the sucker without thinking about it? Same principle applies here, I’d think.
A better defense mechanism is the fact that most baddies would be unable to knock her out in the first place, if she was phased. 
SuperNova and phouka have the right of it, as far as I can recall. The whole “her natural state is phased” bit was a sub-plot of one particular storyline, due to the injury she received in combat.
I think I recall her being held in some kind of pneumatic tube looking device, that helped keep her from “drifting apart” when her condition was at its worst. I don’t remember one way or the other if she couldn’t breathe while phased, but I’m pretty sure she did have to hold her breath when passing through any solid object, including the ground.
Oddly enough, she has one of the most awesome destructive powers ever revealed - she can phase and then destroy almost any matter with ease. She never actually does this on living being because she’s not evil, but she can blow up things with ease.
Actually, if you look around in Yahoo Groups or the web, there are more than a few example of fan art, in which Shadowcat loses her garb.
Not that I ever look at such things…
IIRC, in the Mekanix mini-series, there was a scene where she undressed by phasing herself but not her clothes through the bathroom door. (being Marvel Comics, not Vertigo, you don’t actually get to her her unclothed.)