Why do good floppy disks go bad?

I had one of those Japanese Koan moments recently when someone asked me "Why do floppy disks stop working?" and I realised I... didn't actually know. I thought I'd throw it to the group and get some theories/proofs. Let's work on the assumption we're talking about 5.25" and 3.5" disks. Several guesses: - Repeated use slowly wears away the magnetic media layer on the mylar. - When left in an unprotected state, or a poor environment, damp, mold and dust can damage the surface, either degrading the magnetic layer or causing the gap to shrink enough that the drive head physically damages the disk? - Quantum fluctuations in the state of the universe, caused by millions of mostly non-interacting particles passing through a disk in any given minute, alter the magnetic spin of the ferric atoms causing gradual data loss over time (mostly tongue-in-cheek) - Given the lack of use of most floppy drives they themselves pick up 'gunk' and on first reading a diskette after a long time of disuse damage it. It _seems_ like when you put a 3.5" disk down for ten years and pick it back up, a disk that used to work fine no longer does. Of course, after ten years, it could be your own memory that's failed. Dare I ask, what's the consensus? - JP