Mister Mxyzptlk

Mxyzptlk first appeared on the Post-Crisis Earth in Metropolis posing as a dashing young man named Ben Deroy, who convinces Lois Lane to marry him. Superman quickly learns that he's really an imp from the 5th Dimension. The imp agrees to go home, undoing everything he has done magically, if Superman can get him to spell, type, or say his name backwards. He then proceeds to make up a name for himself on a giant typewriter (brought into existence from an advertisement on a billboard), choosing "Mxyzptlk" as he figures it will be nearly impossible for Superman to accomplish.

After putting up with Mxy's antics, Superman grabs the giant typewriter where Mxy left it and defiantly said that the imp could never type the name forwards. Mxy used his magic to type the same exact keys—but Superman had rewired the typewriter, so Mxy had to go home. After he was gone, some scientists theorized that he would not be able to return for 90 days.