According to that myth, when his mother Thetis dipped him in the river Styx as an infant, she held him by one of his heels leaving it untouched by the waters and thus his only vulnerable body part.
ɐkˈoːɹdɪŋ tə ðæt mˈɪθ, wˌɛn hɪz mˈʌðɚ θˈɛɾiz dˈɪpt hˌɪm ɪnðə ɹˈɪvɚ stˈɪks æz ɐn ˈɪnfənt, ʃiː hˈɛld hˌɪm baɪ wˈʌn ʌv hɪz hˈiːlz lˈiːvɪŋ ɪɾ ʌntˈʌtʃt baɪ ðə wˈɔːɾɚz ænd ðˈʌs hɪz ˈoʊnli vˈʌlnɚɹəbəl bˈɑːdi pˈɑːɹt.