What was Achilles's only weak spot?

In Greek mythology, Achilles or Achilleus was a hero of the Trojan War, the greatest of all the Greek warriors, and is the central character of Homer's Iliad. He was the son of the Nereid Thetis and Peleus, king of Phthia. Achilles was invulnerable in all of his body except for one heel, because when his mother Thetis dipped him in the river Styx as an infant, she held him by one of his heels. Alluding to these legends, the term "Achilles' heel" has come to mean a point of weakness, especially in someone or something with an otherwise strong constitution. The Achilles tendon is also named after him due to these legends.