As originally conceived and presented in his early stories, Superman's powers were relatively limited, consisting of superhuman strength that allowed him to lift a car over his head, run at amazing speeds, and leap one-eighth of a mile, as well as incredibly tough skin that could be pierced by nothing less than an exploding artillery shell. The character's creators, Jerry Siegel and Joe Shuster, compared his strength and leaping abilities to an ant and a grasshopper. When making the cartoons, the Fleischer brothers found it difficult to keep animating him leaping and requested to DC to change his ability to flying. Writers gradually increased his powers to larger extents during the Silver Age, in which Superman could fly to other worlds and galaxies, and even across universes with relative ease. He would often fly across the solar system to stop meteors from hitting the Earth, or sometimes just to clear his head. Writers found it increasingly difficult to write Superman stories in which the character was believably challenged, so DC Comics made a series of attempts to rein the character in. The most significant attempt, John Byrne's 1986 rewrite, established several hard limits on his abilities, such as that he barely survives a nuclear blast, and his space flights are limited by how long he can hold his breath.