-40 C is the temperature at which mercury freezes, making most thermometers useless. When I was a kid in Moscow, Russia, -35 C meant that we didn't have to go to school. -40 C was even more fun - it felt like an entire winter break.
At temperatures like these I remember being clothed in 2 pairs of pants, 3 sweaters, 2 hats, and an incredibely heavy fur coat. Movement was a problem. It also looked very funny: imagine stuffed animals trying to jump across mounds of snow in the dead of winter.

-40 C is the point at which the Celcius (or Centigrade) and Fahrenheit scales converge. It's also pretty damn cold. Canadians have been known to wear gloves and toques when it gets this cold. Also your snot freezes in your nose.