Why does Air gets thinner at higher Altitude

I find this sort of discussion absolutely fascinating and eagerly await all answers, whether they be funny or educated. Scientists would be annoyed by this, no doubt, because they like everything tied up in neat little packages, but to me this kind of question inevitably leads to a sort of which came first the chicken or the egg dilemna. Any reasonable sounding answer seems to be tied to one's preexisting acceptance of some other "fact" which is based on yet another "fact" but all these facts sometimes seem to me like a house of cards. Pull one out and the whole theory falls on its face. I'd just love for somebody here to prove me wrong!

According to weatherquester.com, 'less mass above squeezes it less, so the air is less dense.'

Say! that's a good question, so I came in to see if anybody answered it.. and I see right off the bat, the first poster did. Kool. I didn't know what the answer was, but now I do!

hmmm I do wonder why.. it is true when we go up into the mountains it becomes more difficult to catch your breath. I wonder if it has anything to do with gravity - the pull of the earth's surface?