I read an unofficial biography on superman's powers where it is suggested that his invulnerability works something like this:
His cells collects solar energy which creates a protective aura arround his body, like a force field. This explains the good condition of his costume. However, they want to simeltaneously pass this off as the source of his indestructibility.
Superman protected by a solar force field/aura?



??
Needless to say that I disagree with this silly theory. It has always been my understanding that superman's actual physical body is indestructible. This includes his internal organs: bone, tissue, lungs, heart, the whole shebang right down to his individual cells.
I figured it to be a combination of his cells's ability to absorb solar energy as well as his super dense molecular structure. That is, his Kryptonian cells's ability to absorb almost infinite amounts of yellow sun radiation combined with it's natural dense structure resulted in his cells binding together so strongly that they cannot be disrupted at a physical level as long as they keep on being radiated.
I also never liked the idea that his cells could store the energy for only up to three days (read that somewhere in some issue long ago). It seems to me that his cells should be able to retain this energy for much longer since they are such effective "solar batteries". For 6 months maybe?
Your comments on this will be interesting.