Today, “kryptonite” mostly works entirely absent Superman’s hero cape. It’s a term denoting a moral weakness, a character flaw. The idea being that humans are powerless in the face of this or that vice or guilty pleasure. Who cares why? It just sounds cool when people describe their shortcomings this way, confiscating Superman’s assumed virtue for themselves: “cigarettes are my kryptonite.” This kryptonite is metaphorical, a weaker, abstracted copy of a space rock that serves as a totem. But it makes more sense as a metaphor than as an object that’s just a cheap, flimsy deus ex machina.