Wasabi is not the same as horseradish, says Bill Poser in an entry heavy on lexical analysis. But Westerners can’t tell the difference because we’ve never been given the real thing:

The real thing is expensive, and for it to be any good, it has to be freshly grated from the root. The result is that as sushi has become more popular, more and more of the “wasabi” served in the United States has been fake. It isn’t wasabi: it’s horseradish with green dye. Many people don’t know the difference between wasabi and horseradish because in their experience there isn’t any.

(via Languagehat)