Why is Superman weakness Kryptonite if he is from Krypton?
(This idea was further developed in the Elseworlds series Kingdom Come, when Luthor reveals that the older Superman's absorption of solar radiation over the years rendered him immune to Kryptonite.)
It's radiation that makes Superman weak. The prevailing theory in geekdom is that kryptonite can kill Superman because it disrupts his solar panel-like energy absorption. Without energy from the Sun, he weakens and eventually dies. Though kryptonite is a fictional mineral, the way it interferes makes sense.
Green Kryptonite
Green Kryptonite makes Superman weak and can even kill him. But, Kryptonite comes in many variations—red, gold, blue, white, etc. All affect superman differently.
Lex Luthor
Thanks to DC Kids, here's a look back at a scene from “Superman: The Animated Series” in which Lex Luthor discovers Superman's weakness after the Man of Steel is suddenly weakened by a mysterious rock when he attempts to stop a robbery at the museum.
Weaknesses. Kryptonite: Like all Kryptonians, even while empowered, Zod is vulnerable to the exposure of Kryptonite, a radioactive mineral from his home planet Krypton.