Why does kryptonite make Superman weak?

Classic heroes, even seemingly invincible ones, must have one weakness. We call it the “Achilles heel” because the defining example of a hero's weakness is in the Iliad. Myths say that the warrior Achilles' mother, Thetis, dipped him in the river Styx to make him invulnerable, but she missed the heel by which she held him. When Achilles is shot in the heel with an arrow, it pierces his flesh and kills him.

Kryptonite is Superman's vulnerable heel—it's not as ubiquitous as the “Achilles heel,” but “Kryptonite” has also become synonymous with “one weakness” in the pop culture lexicon. (Take the rock song “Kryptonite” by 3 Doors Down.)

Superman needs the weakness that Kryptonite provides because a completely invincible hero isn't fun or engaging to watch (No one tells Mark Grayson). In an action narrative, if nothing can defeat your protagonist, there is no tension. Don't believe me? Take the words of the late “Superman” editor Dorothy Woolfolkwho is widely credited with introducing Kryptonite to comics. She said that without a weakness, Superman would be “boring”.

Kryptonite would be a particularly effective storytelling tool for a radio series; you could end one episode on a cliffhanger with Superman trapped and weakened by Kryptonite, ensuring people tune in next time to see how he escapes. When it was brought to the comics, it ensured that a normal man like Lex Luthor could possibly be Superman's archnemesis; all Lex needs is a piece of kryptonite to level the playing field.

Many other Superman villains use kryptonite gimmicks. Metallo is a cyborg powered by Kryptonite. Titano, a King Kong-like ape, can shoot kryptonite beams from his eyes. Bloodsport played by Idris Elba in The Suicide Squad uses ammunition forged from kryptonite to make Superman less durable. These villains can harm Superman, so he in turn has to expend care and energy to fight them, which makes his triumph over them feel more earned.

The series “Smallville”, which followed the young Clark Kent (Tom Welling), combined “Superboy” and “Buffy The Vampire Slayer”. The “Smallville” villains of the week weren't demons, but normal people empowered by the kryptonite meteors found around Smallville; these one-time detractors were collectively known as the “meteor freaks”.

“Smallville,” which relied heavily on Kryptonite as a MacGuffin and superpower-giver, showed the same problem as the Silver Age (50s-60s) “Superman” comics—use Kryptonite too much or make it too versatile, and it goes from easy to tension on the narrative crutch.