How cold is 40 below?

For years now I've had a little joke that I've played on friends... particularly when the weather gets really cold. I'll make a little comment about how it feels like it's 40 degrees below zero. And then stress that I mean Celcius. Then I ruminate about what that would work out to in Farenheit. The conversion process is fairly simple... if you're starting with Celcius, you multiply by 9, then divide by 5. After that, you add 32 and you're done.

So I explain the process to my friend slowly enough so he can do the math as I recite the equation. Finally, I wait until I see the puzzled look on his face and he starts over. It never fails, especially if he's using a calculator. Incidentally, this only works with 40 below zero. I'm so evil.