“Wish I could explain my strange reaction to that meteor!” Clark Kent wonders aloud. “Why do I get weak every time I come within five feet of it? And Krypton… Why did I keep repeating that word, over and over again? Krypton… What has the word Krypton to do with me? Sounds familiar, but I… just can’t place it! I must find out, because unless I’m very much mistaken… Krypton is the key to this whole strange business!”
You see, back in the old days, little Kal-El didn’t arrive on the planet Earth with a crystal library full of ancient knowledge and a hologram of his dad to explain how to use it; the kid just crashed, and it was up to the passing motorists to figure everything out from scratch.
So in 1943, when the Adventures of Superman radio show decided that they wanted Superman to know where he came from, they invented a meteor and called it Kryptonite, and then they put it in a drawer and forgot about it for another two years.