The next time you open the plane's window shade at a stewardess's request, do it happily, because it's for your own damn safety. Tech Insider asked one flight attendant to enlighten passengers as to why this is a necessary procedure in getting a plane on and off the ground, and it makes a whole lot of sense. The stewardess explains:

If there’s an emergency, we have to be able to look out the window to assess outside conditions. If there is debris, fire, or water blocking the window, we won’t use that exit and will direct people elsewhere. It may seem like a small thing, but in an emergency, every second counts. You don’t want to have to fiddle with a window shade when you’re trying to safely evacuate a plane in 90 seconds.

Sources: Tech Insider