Will o' the Wisp

I don't know why the word "ephemeral" showed up in my brain this morning, but it did, and I spent a good ten minutes trying to pick it apart and figure out where it might have come from (I was in the shower, which doesn't require a whole lot of brainpower). "E-" is usually a shorted form of "ex-", "out of", in Latin words, but the "-ph-" suggested it was Greek, and I couldn't think of a single related word that seemed to have anything to do with the fleeting nature of ephemerality, so after taking a few wild stabs and basically just making stuff up I eventually gave up and did the research. It isn't as gratifying as figuring it out yourself, but it's more likely to be correct.

The only thing I got right was that it is in fact Greek, which was a pretty obvious assumption. As for the actual construction of it, I never could have guessed it, because it's formed from "epi-", "on" (as in "epicentre"), and "hemerai", "day", and it originally entered English as the phrase "ephemera febris", a fever that lasts a single day, and only later was enlarged to mean anything that is short-lived. The reason I couldn't identify any other common English words with this root is that there aren't any.