In Superman #19 (1942), Clark Kent and Lois Lane go to a movie theater and watch one of the Paramount Superman cartoons. Since the cartoon depicts Kent changing into Superman, the "real" Clark repeatedly has to distract Lois to keep her from seeing it. The weird thing was, he did not seem worried about the other audience members, only Lois, as if she were the only one who did not know his secret identity. Or, as if she were the only one who could not be allowed to find out.The story was reprinted in Superman #183 (aka 80-Page Giant #G-18) in 1965 (cover date Jan. 1966). Then, it was billed as DC's first "imaginary" (i.e., non-canonical) story.