If you always do well when you play along with "Jeopardy!"; if you're always picked first for a "Trivial Pursuit" tournament; if you're the "go-to guy" at your workplace for the correct spelling of arcane vocabulary, then you've probably at some point been asked some variant of the following question:
"How did you know that?!?"
A fair question? Not really. Oh, in some cases it's more than fair--it's downright necessary. If one of your colleagues walks up to you and proudly recites your social security number, you are not only justified but arguably also obligated to ask the question. And you have every right to a specific answer. For the most part, though, the question is probably unanswerable. Are you really supposed to remember the circumstances under which you learned the name of the plastic part of a shoelace or the periodic table abbreviation for tungsten or the Korean word for 'giraffe'? And even if you do remember, does it really matter HOW you know something?
And therein lies the problem with the question. When the question is asked, it is usually not in the spirit of investigation, as in the social security number example. Nor is it asked in a sort of reverential tone of deep respect and admiration of your knowledge. In fact, the questioner is not really asking "How did you know that?" at all.
He's really asking, "WHY do you know that?"
Because underlying the question is an assumption that the knowledge is so obscure, so off-the-beaten-track, that there is something sinister in your possession of it. A typical response is, "Oh, I don't know, something I picked up somewhere" or the slightly more self-aggrandizing, "I read a lot" (often said with a small shrug and shake of the head so as to deflect accusations of braggadocio). But why such a display of false modesty (as we've established that all modesty is)? If you don't know "where" you know something from, say so. If you happen to remember that you acquired some factoid from high-school chemistry or The Book of Lists or an overheard conversation, you may choose to provide the provenance if you wish.
But the most appropriate response? How do you know that?
"I don't know how I know it, I just do. And now, thanks to me, you know it, too, so I hope you'll give me proper credit when you get to pass it along."
And if they ask the more honest and direct question--the one they really want to ask--"Why do you know that?"
"No particular reason. Why don't you?"
By the way: Aglet, W, and Girin.