In order to deceive Nazi Germany about the real target of the D-Day invasion, the Allies dropped literally tons of aluminium foil strips (then codenamed “Window”, now known as Chaff) from aircraft to create the appearance of a large fleet moving across the English Channel towards Calais on German Radar screens just before the actual invasion started

Thanks to Bletchley Park reading German ciphers, Allied high command could watch the Germans fall for the deception almost in real time.