What is Superman's greatest weakness?
For those of you who aren't total nerds, let us give you a bit of a backstory. Kryptonite is a radioactive fragment from Superman's home planet, Krypton. Other than that, what Kryptonite is or does depends on what genre your Superman comics are from. You see, just before the explosion that destroyed Krypton, Kryptonians Jor-El and Lara outfitted their son Kal-El's birthing matrix for space travel (like a traveling baby pod). This futuristic space-stork traveled through space, pulling fragments of Krypton, made radioactive in the explosion. This radioactive debris became known as Green Kryptonite, or simply kryptonite, and it is deadly to super-powered Kryptonians i.e. Superman. Hence, the reason Kryptonite is Superman's, well, Kryptonite. Source: Marvel Comics, DC and How Stuff Works