Kryptonite Vulnerability

There is something that Superman keeps in a lead-lined box in the deepest room of the Fortress of Solitude.

He visits it rarely. When he does, he does not open the box. He simply stands near it, and he reminds himself of something important.

Inside the box is a piece of Kryptonite.

Kryptonite is what remains of Krypton — small chunks of the planet, scattered through space when the world exploded, some of them finding their way to Earth over the long years of Superman's life. They glow. The most common kind glows green, a soft and terrible green that is beautiful the way a thunderstorm is beautiful — from far away, behind glass. Up close, if you held a piece and Superman stood within a hundred feet of it, you would see something that looks impossible: you would see Superman get weak.

Not just tired. Weak in a way that starts in his bones and moves outward, a pain like every muscle forgetting what it is supposed to do. The powers do not fade slowly — they fall away in pieces. The hearing first. Then the strength. Then the speed. Then the invulnerability, and this is the part that is the most frightening, because Superman is used to moving through the world as something the world cannot hurt, and Kryptonite reminds him very suddenly that this is not always true.

Lex Luthor knew about Kryptonite. Of course he did — Lex made it his business to know everything. He had spent enormous effort acquiring even the smallest samples, had them analyzed and catalogued and turned into devices that he stored in his private laboratory under LexCorp Tower. He considered this the great equalizer. The one advantage.

Superman did not begrudge him this. He understood it.

The Kryptonite was Lex's fear made solid. It was the thing that said: this alien is not invincible. This alien can be hurt. And Lex needed to know that, because without it, the world felt — to him — like a place where he could never fully trust what he had built.

Superman thought about this. He thought about it for a long time.

Then he decided to do something that most people considered very strange.

He gave Batman a piece of Kryptonite.

Not a large piece. A small shard, the size of a coin, sealed inside a lead case, with a note that said: "Keep this. Use it if you ever need to."

Batman held the case in both hands and looked at it. He looked at Superman. "You know what this means," he said carefully. "This means I could —"

"Yes," Superman said. "I know."

"Why are you trusting me with this?"

Superman was quiet for a moment. "Because I trust you," he said. "And because the day I believe that no check on my power is needed is the day I become exactly what Lex thinks I am." He looked at Batman steadily. "I'm not perfect. Nobody is. If I ever lose control — if something ever goes wrong in a way I can't fix — you'll know what to do."

Batman was quiet for a long time.

Then he said, "I'll never have to use it."

"Probably not," Superman agreed.

"But I'll keep it."

"I know."

The lead case went into Batman's utility belt, in the compartment he never opened. And Superman went home to Metropolis, where the yellow sun was warm on his face and the city was full of ordinary noise and ordinary beauty and everything was fine.

He was not invincible. He had never pretended to be. But he was surrounded by people who understood both his strength and his limits, and who stood beside him anyway.

In the Fortress of Solitude, the lead box sits on its shelf. On nights when he visits the Fortress, Superman sometimes stands near it — not to open it, just to remember.

A hero is not defined by the absence of weakness. A hero is defined by knowing their weakness, telling the truth about it, and going out to do the work anyway.

The box is closed. The box is sealed. The work is not.