The Facts on File Encyclopedia of Word and Phrase Origins
顾名思义，讲词源 (etymology) 的，可以当作百科全书 (Encyclopedia) 来看。
试举一例：
Achilles’ heel. When he was a baby, Achilles’ mother, the
goddess Th etis, dipped him into the magic waters of the river
Styx to coat his body with a magic shield that no weapon could
penetrate. However, she held him by the heel, so that this part
of his body remained vulnerable. Paris learned of his secret
during the Trojan War, shooting an arrow into his heel and killing
him. Achilles’ heel has since come to mean the weak part of
anything.