Question:
I'm sure this has been asked before, but I couldn't find an answer. Why does my bottled "pure" drinking water have an expiration date on it? What's going to happen if I put the bottle in the fridge for the next year and a half and drink it the week after it 'expires'? I was thinking maybe something in the plastic they make the bottle out of might contaminate the water, but the same date shows up on glass bottles too.

Answer: Just guessing, but it may have something to do with how good the bottle is sealed. There is bound to be some type of transfer of air between the inside of the bottle and the outside (around the cap; I doubt the bottles are 100% air tight). It is probably very slow, but after some period of time some amount of impurities would have managed to get into the bottle and into the water even without it being opened, which means past some point it is no longer pure (to whatever standard of "pure" the bottled water company uses

Arresting the foolhardy who drink expired bottled water is just another part time job for the mattress tag patrol. You'll know how effective they are: you're just thinking about it, and already - well, look for yourself. See that mysterious van parked down the street from you?

Flavored water might just have something in it that could really expire, although I kinda doubt that it would do bad things to you. I think most expiration dates (like the freshness date on soda pop) are marketing schemes, with barely any connection to reality.