Comic Book Questions Answered – where I answer whatever questions you folks might have about comic books (feel free to e-mail questions to me at email@example.com).
Reader Gerry D. wrote in to ask why Kryptonite has a negative effect on Superman when it is just made up of pieces of Superman's own home planet?
Really, the answer for almost everything in comic book history that seems like it doesn't necessarily make sense ultimately comes down to "because we said so." That's where these things begin and then sometimes writers will go back and try to come up with an explanation that fits the facts, but that is not the real reason why, the real reason is because someone decided that that was the case.
Kryptonite originally appeared on the Superman radio series in the early 1940s. Here's the hilarious thing about kryptonite. The whole idea is that they come from Krypton, right? Well, you see, for the first decade of his comic book existence, Superman did not even KNOW he was from Krypton! That's just logical enough, though, right? Later comic book stories (and films) have come up with ways to explain why Superman would learn his origins earlier in his life, but generally speaking, if you just land on earth as a baby in a rocketship, how in the world do you know where you cam from?
Not only that, but Superman's early years were very forward-thinking. There was not a whole lot of time spent on Superman's past. He did not even get a full origin until his 10th anniversary, when Bill Finger gave a full origin for the Man of Steel. A year later, Finger expanded that origin to allow Superman to learn the truth. This came after a piece of a meteorite was making Superman terribly weak when he was around it (it was used as the jewel in a Swami's head dress, so the Swami believed that it was his own mental powers that were causing Superman to weaken when he was around him). So Superman investigated the meteorites and then traced their path back in time (because, you know, Superman used to be able to travel back in time at the drop of a hat) and he learned that the meteorites came from his home planet, Krypton.
So the initial story is that, when Krypton exploded, the entire planet was fused together to form a new compound and that new compound was Kryptonite. So it was not Krypton itself, but rather what Krypton transformed into when it exploded. So it is not that Superman is having a problem with pieces of his own home planet, but that his home planet happened to be changed into something completely different from the explosion.
That idea (that Kryptonite was a compound formed from the explosion of the planet Krypton) was mostly kept consistent over the years. However, there were some interesting twists on the concept from time to time....