An evil thing that happens when you do not re-apply the funky shiny stuff called Birkenstock Cork Life on the cork part of your birks While covered in the sealant, birkenstocks are just about invincible; fail to reapply it, though, and if you get them wet, or just wear them too much, or live where it is really really hot, or any combination of the above, the cork will start to decay to the point where the sandals are not repairable without being resoled entirely. Yeah, the stuff is a little more expensive than it should be. (It's still pretty affordable though; between five and ten bucks a bottle.) But conversely yes you are a complete moron if you do not buy it. A bottle of Cork Life is much cheaper than new cork, or heaven forbid a new pair of Birkenstock sandals.