I bet there's a lightswitch in the gallery too. Is that art?
What about the flooring? The lighting arrangement? Is that still art?
How about the seat that the museum/gallery staff sit on while they watch you don't touch anything?
How about the staff themselves?
And, yes, at some point in time EVERY SINGLE ONE of those things may be considered art in a particular instance, but it doesn't make all lightswitches art, so it's not a defining characteristic (even ignoring the self-fulfilling prophecy of anything in an art gallery somehow becoming art, which would seem to preclude anything *outside* the gallery ever being considered art).
You can't define art by its location any more than you can define it by it's origin ("Oh, yes, Turner made this so it must be art", say the people gathered around a pile of his excrement), age, history, technique, size or anything else. There is no one defining characteristic, no matter how complex, that makes something universally "art". As such it's entirely subjective and has been for the last 100 years or so (interestingly, before that, pretty much everyone agreed on what was art and what wasn't).
If it's subjective, my own personal assertion that anything that doesn't take skill to reproduce isn't art is equally as valid as yours that a water stain is art. If you don't recognise that both assertions are equal, you're just being pretentious unless you can come up with a definition of what art is that everyone (or even a majority) accepts.
Mona Lisa? Art, in my opinion, because it would take immense skill to reproduce the item in question by hand with only a paintbrush and some colours (which is how it was originally made). There's no way I could do it in a hundred years of trying. Even a decent artist would be unable to capture the same magic. Almost all of the classical artists, for everything from chapel ceilings to sculptures to paintings, fit that profile.
"Modern art" doesn't. Any idiot can, quite literally, reproduce using identical techniques those "pieces" - hell, children often do a better job every day in school (see my example above about the Olympic posters, for example) - and there have been tests and trials where even the most experienced art-lover can't distinguish between a child's drawing and the work of some modern artist that they are familiar with. I think you'd spot a child's version of the Mona Lisa, though, against the real thing and if you *couldn't*, I think it's fair to say that child was a good artist.
You can be as pretentious as you like and the whole art world since about the 20's has tried to form a clique by the simple precept of saying that nobody else understands them, even when they are caught worshipping the scrawls of an actual child rather than the artist they think made it, and that anyone who doesn't understand these unwritten, unspoken rules is somehow stupid. In that instance, I'll be on the side of stupid, thanks, as will vast proportions of the world's population.