During WWII, President Franklin Roosevelt asked an aid to write instructions for darkening government buildings for an air raid. The aid wrote: 

Such preparation shall be made as will completely obscure all Federal buildings and non-Federal buildings occupied by the Federal Government during an air raid for any period of time from visibility by reason of internal or external illumination. Such obscuration may be obtained either by blackout construction or by termination of the illumination.

President Roosevelt rewrote the memo to say: 

In case of an air raid, turn out the lights. If you have to keep working, put something over the window.