In mathematics, the letter 'x' is used to denote an independent variable. It also once described the unidentified radiation in x-rays, and more recently was shrouded in mystery in the TV show The X Files. But how did the letter become the symbol that commonly represents the unknown? Terry Moore's TED talk takes us back to the origin of "the unknown thing," explaining how its translation from Arabic to Latin was facilitated by Spanish phonetics and the Greek alphabet.