Why every hero has a weak spot

Thetis dips Achilles in the Styx, holding him by the heel - and the heel stays dry. Siegfried bathes in the blood of the dragon he killed, but a linden leaf sticks between his shoulder blades, leaving the skin beneath it ordinary. In both cases, the weak spot is not a flaw in the hero, but a trace of the procedure: the place where he was held and the place that was covered.

The same gap can be in a contract rather than a body. Frigg secures a vow from every thing in the world not to harm Baldr, but mistletoe seems too young to her to promise anything - and remains the only thing unbound. In the Shahnameh, Esfandiyar is invulnerable everywhere except his eyes; retellings explain that he closed them during the ritual. The logic is the same: invulnerability in myth is not a trait but work, and every job has a place it did not reach.

You stay with a story while you do not know how it will end. A completely invulnerable hero removes that question: a fight with him is a victory report written in advance. One vulnerable point brings death back into the story, and with it, fear for the hero. It still works today: writers give modern unbeatable characters kryptonite for much the same reason.

The second move is about who knows the secret. An enemy almost never finds the weak spot - someone close to the hero gives it away. Loki questions Frigg herself about mistletoe. To keep her husband safer, Kriemhild embroiders a cross on his clothing - and shows Hagen where to strike. Vulnerability turns a fight into betrayal.

That may be why the heel became attached to Achilles. In Homer, he is not invulnerable at all: in Book 21 of the Iliad, his forearm is cut open and bleeds. The bath in the Styx and the dry heel appear in texts many centuries later - most fully in the work of the Roman poet Statius. The detail was added not for beauty: without it, Achilles' death remained just another fight.

If a hero is declared invulnerable, what usually follows is a description of exactly how they were made that way - and the gap is named there too. This is not a storyteller's trick, but a promise: the gun is shown in advance.