Where Does The Expression "Achilles' Heel" Come From?

In Greek myth, the hero and warrior Achilles was the son of a nymph, Thetis. His father was mortal, so Thetis determined to give her son immortality. She dipped the baby in the River Styx (in myth, this was the river over which the souls of the dead travelled to the underworld; it could give immortality to a living person.) This made him invulnerable to all attack, but as Thetis had to hold Achilles by one heel, this part wasn't dipped. In later years Achilles became a famous warrior on the Greek side in the Trojan war. He killed hundreds of Trojans including Hector, eldest son of the Trojan king. In the end he himself was killed by another of King Priam's sons, by a poisoned arrow which struck him in the heel.
Even now, we say "Achillles' heel" to mean a person's one vulnerable point. There is also a part of the foot known as the Achilles tendon.

Achilles was a character in I believe greek mythology. During the battle of Troy, he apparently received a blow to the back of his foot, just above the heel, which then in turn crippled him in battle.  You can feel your "achillies tendon " on the back of your foot/heel. It is stretched to tight it almost feels like a bone.