It is the only substance that can weaken Superman aka Clark Kent. Superman is a comic book superhero and does not exist in real life nor does kryptonite.

it's what u use in our fist to kick the $#!7 out of superman!

Kryptonite is a mineral from the Superman mythos, originating in the Superman radio show series. The material is usually shown as having been created from the remains of Superman's native planet of Krypton, and generally has detrimental effects on Superman and other Kryptonians. The name "Kryptonite" covers a variety of forms of the substance, but usually refers to the most common "green" form. The word Kryptonite is also used in modern speech as a synonym for Achilles' heel, the one weakness of an otherwise invulnerable hero.

krypton A colorless element that is one of the six inert gasses; occurs in trace amounts in air atomic number 36

Krypton is a chemical element (Kr 36). It's name is used for the fictional planet Krypton (where Superman comes from). Kryptonite is very common on this plaet. If Kryptons (people from the planet Krypton) leave, they gain their superpowers. But they are becoming weak again if there is any Kryptonite around. The term 'crypton' is used in encryption, too. D., usually not talking weird stuff like that. Who hates those stupid superhero-comics. http://www.domasla.de/comics.htm