When a waiter at a restaurant comes by with pepper or Parmesan cheese, he says, "say when" and starts putting it on your food. Many people will say "OK" or "that's enough," but it seems that the customary answer is "when". How did this become the customary answer? Did everyone decide to make the exact same joke, until it became commonplace. I thought it was funny when I was a kid, but even now as an adult, I still say it.
For evidence that this is indeed commonplace, see http://www.gocomics.com/broomhilda/2012/06/20. The answer "when" is used and is external to the joke of the strip. But even so, I'm curious as to just how commonplace this really is. I see it a lot in the US. Is it used elsewhere in the world?
(by the way, I'm new to asking questions on this site, and I have no idea what tags to put on this question)