Classic heroes, even the seemingly invincible ones, must have one weakness. We call this "Achilles' heel" because the defining example of a hero's weakness is in the Iliad. Myths say that the warrior Achilles' mother Thetis drowned him in the river Styx to make him invulnerable, but she missed the heel she was holding him to. 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 "Achilles's heel," but "Kryptonite" has also become synonymous with "single weakness" in the pop-culture lexicon. (Take 3 Doors Down's rock song "Kryptonite.")

Superman needs a weakness, which Kryptonite provides, because a completely invincible hero is not fun or engaging to watch (no one tells mark grayson). In an action narrative, if nothing can defeat your protagonist, there is no tension. You don't believe me? Take the late Superman editor Dorothy Woolfolk at her wordwho is widely credited with bringing Kryptonite into comics. Without a weakness, she said, Superman would be "boring."

Kryptonite would be a particularly effective storytelling tool for a radio series; you can end an episode on a cliffhanger with Superman trapped and weakened by kryptonite, ensuring that people tune in next time to find out how he escapes. When he switched to comics, he made sure that a normal guy like Lex Luthor could be Superman's archenemy; 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 from his eyes. Bloodsport, played by Idris Elba in The Suicide Squad uses ammo forged from kryptonite to make Superman less bulletproof. These villains can harm Superman, so he in turn must expend care and energy fighting them, which makes his triumphs over them feel more earned.

The television series "Smallville", which followed a young Clark Kent (Tom Welling), combined "Superboy" and "Buffy The Vampire Slayer". Smallville's villains of the week weren't demons, but normal people who were given powers by the Kryptonite meteors lying around Smallville; these one-time bad guys were collectively called the "meteor freaks".

Smallville, which relied heavily on Kryptonite as a MacGuffin and superpower-giver, exhibited the same problem as the Silver Age (50s and 60s) Superman comics—overusing Kryptonite or making it too versatile, and goes from light suspense to narrative crutch.