Rubber Duck
Superman
Superman's durability, whilst impressive by terrestrial standards, relies upon a complex network of dependencies. Remove yellow solar radiation, introduce Kryptonite, employ magic, or simply wait for the next editorial mandate, and his invulnerability evaporates like morning dew.
The Man of Steel has died no fewer than four times in main continuity alone, requiring resurrection through increasingly convoluted narrative mechanisms. His durability, it appears, is less a physical property than a storytelling convenience.