From June and July 1939 Superman newspaper strips by Jerry Siegel and Joe Shuster.
These two panels - parts of strips from two different days, but from the same storyline - highlighted something for me in a very clear way. Superman and Lois are both trying to uncover shady doings at a state orphanage. Both use exactly the same tactic to investigate: sneaking in through a window. Only one of them has superpowers.