Ghost Butts

My roommate and I spent our unusually free Wednesday night watching the movie Ghost. I had her assurance that despite the name, the movie could not possibly be classified as "horror", and it was, in fact, quite funny. (I refuse to watch horror movies. I think the last "scary" movie I saw was Kucch To Hai when I was 13. Among my group of maybe 10 friends, no one else found it even remotely creepy; I, however, was shrieking and leaving nail-marks on my friends' arms through most of the movie.)

Anyway, Ghost wasn't a bad way of spending some unexpected free time, even though I realised halfway through the movie that I've seen it before. And it raised an interesting question: how do ghosts stand?

In all the movies I've seen that feature ghosts (and yes, I do accept that there haven't been too many), the ghosts can never touch anything. They can't throw rocks, they can't pick up a glass, they can't poke their loved ones, they can't beat up enemies threatening loved ones. If they're lucky, they can be seen or heard (or both), but the touching is always a problem. Then how is it that they can always stand without falling through the ground? Sam (in Ghost) does learn how to throw and hit and poke, but he was standing on the ground long before he learn to "focus his emotion" onto physical objects and pick them up. How was that happening? Even if we say that the earth is solid, you can't fall "through" it the way ghosts "go through" doors and other objects, how does a ghost stand on a train, or a bridge, or the second floor of a house without falling through and hitting (so to speak) the ground below?

To carry this on, how do they sit? Ghosts are always sitting on chairs and tables and sofas. How do their ghostly butts just not fall through? How do ghosts lean on stuff while making sarcastic comments? How do they kneel on the floor while desperately pleading the aforementioned loved ones to hear them?

My roommate thought that maybe it's because they pass through objects only when they want to. To which I triumphantly pointed out that most of the first half hour of Sam's post-death appearance (in Ghost) was spent by him trying to poke and hit every object he could see, to prove that he was still real; most of that half hour was a pointless exercise. So while he desperately wanted to touch something, he could not; his hand would simply go through it. So why didn't his butt go through the sofa or the chair?

I did try googling this. But if I type "ghost sit", google thinks I've misspelled "site" as "sit", or gives me some video of a ghost sitting in a corner. Typing "ghost stand" tells me what ghosts stand for, but isn't particularly illuminative of the secret-of-physically-standing issue. My research skills, however, aren't all that great (a fact that shows in term papers and essays), so maybe someone can help me out? Is this a glitch that movies have just not able to fix or explain away, or is there some ghostly secret that I don't know about?