Achilles' heel

When the hero Achilles was born, his mother Thetis was told he would die young. Desperate to cheat that fate, she carried him down to the underworld and dipped him into the black water of the River Styx, whose current made mortal flesh proof against any blade.

She held him, as any mother would, by the heel. The water closed over every inch of him but the small patch her fingers covered — and there, unseen, the boy stayed as mortal as the day he was born.

Achilles grew into the greatest warrior of his age. Through ten years of war at Troy he walked out of battles no other man could survive, until the whole Greek camp believed him untouchable.

But Paris, guided by the god Apollo, loosed a single arrow — and of all the places it might have struck, it found the one dry patch of skin on his heel. The invincible hero fell to the smallest of wounds.

Ever since, an Achilles' heel has meant exactly that: the one hidden flaw that can bring down all the rest of a person's strength.