I was at a cafe yesterday getting some tea and the guy at the counter put a plastic sippy cap on my tea. I got on his case a little because he put it in another paper cup instead of putting one of those cardboard ring things around it (which is supposed to save paper or something); anyways, while I was moaning about it I realized the plastic sippy cap had an expiration date on it, and the date was 12/16/20.

That's a pretty specific date, you know, too? So, how does a plastic sippy cap go bad? What happens to it?

Cheers,
Lost

I don't think that has to do with the plastic going bad necessarily...I think it is more of a quality control measure.
http://www.epa.gov/grtlakes/seahome/housewaste/house/plastic.htm#composition