Few superheroes have abilities as impressive as those of Superman, but the further back you look in the Man of Steel's history, the more ridiculous his feats become. For instance, there was that one time Superman destroyed an entire solar system...by sneezing.
Published back in 1961, Action Comics #273, titled Superman: The World of Mr. Mxyzptlk, pits the Man of Steel against the meddlesome Mr. Mxyzptlk, an imp from the fifth dimension. This issue was not their first encounter - Mr. Mxyzptlk first appeared in Action Comics #80, published in 1944. Being from the fifth dimension means Mr. Mxyzptlk is not bound by the physical laws of our three-dimensional world and can manipulate reality at will. Despite his god-like powers, Mr. Mxyzptlk is little more than a trickster who normally uses his abilities for practical jokes, like giving the mayor the voice of a donkey or turning the moon into cheese. His favorite person to prank is Superman, but messing with a being as strong as the Man of Steel can be dangerous.
Such was the case in Action Comics #273. While Superman is on his way to visit a local orphanage, he detects a meteor-like object heading towards Earth. He destroys it before it can collide with the planet, but in doing so, causes the object to explode into sneezing dust, which rains down on Metropolis. When Superman inhales all the dust to protect the people, Mr. Mxyzptlk appears, revealing that the meteor was his doing and the dust is magic, which Superman is vulnerable to. Superman feels a powerful sneeze coming on and quickly flies to a distant solar system to let it loose.
Good thing he did, because his sneeze wipes out the entire solar system, which was conveniently uninhabited. To get back at Mr. Mxyzptlk for making him miss his appointment at the orphanage (and, you know, for making him almost eradicate humanity), Superman travels to the fifth dimension and plays some practical jokes of his own while Mr. Mxyzptlk is running for mayor. Superman's pranks cause Mr. Mxyzptlk to lose the election; probably for the best considering what a troublemaker he is.
As absurd as Mr. Mxyzptlk's encounters with Superman are, their conflicts did have a lasting impact on Superman's character. It was through Mr. Mxyzptlk that readers first learned of Superman's vulnerability to magic, a weakness that became crucial in later Superman comics.