Rubber Duck
The rubber duck demonstrates what materials scientists term functional immortality. Its polyvinyl chloride construction resists water damage indefinitely, as one would expect from an object designed exclusively for aquatic deployment. Specimens from the 1970s remain in active service, their squeaking mechanisms still operational.
More significantly, the rubber duck possesses no moving parts to fail, no power source to deplete, and no psychological vulnerabilities to exploit. It cannot be corrupted, mind-controlled, or convinced to doubt its purpose. It simply is.
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.