Reactive Lenses On Line Do Unopened Contact Lenses Expire? If So, How?

Do unopened contact lenses expire? If so, how? - reactive lenses on line

I know that when buying contact lenses have an expiration date printed on them. But how long the life of you? I mean really? They have no food, so I do not really understand why it ends. "Perhaps a chemical reaction? But what reaction? They are not responding, most polymers?

The reason I ask is that I wear glasses, but it would be good to have contacts for things like paintball, scuba diving, water sports, etc. If I had to do, if I used as contacts for such things, that is, it would be on the shelf for a while.