Achilles Heel

There are two different story's to what happened to make Achilles invulnerable.
The more classic story is that there was a prophesy which claimed Achilles would be killed in battle.
So when he was a baby his mother dipped him in the river Styx which would cause invulnerability however his mother was careless and forgot to dipped the one part she was holding which happened to be his heel.
The ironic thing is  because she forgot this Achilles was killed by Paris the brother of Hector who was killed in battle by Achilles.
The saying Achilles heel has come to mean a persons weak spot or their vulnerable spot as was Achilles' heel.