So, I know this has pretty much NOTHING to do with computer security and all but its driving me crazy. I'll just post it here, cause I have seen more intelligent people on this board over the past years than any other.

The other day I was hanging out with my room mate, smoking some *cough* tabacco, out of our *cough* 3ft water pipe. And he wanted me to finish the hit that he could not finish. Of course if you have ever smoked out of a "Water Pipe*", then you know left over smoke quickly goes stale. So i say " No way dude, its all stale. It'll make you cough." and he says there is no such thing as "Stale Smoke" . And he proceeds to challenge me to find scientific PROOF that smoke goes stale.

So heres my question: What change takes place on the molecular level to smoke after X amount of time?

I've searched google but found nothing really.... anyhelp would be appriciated.