I was just speaking to my friend’s Grandmother who was a child in Denmark during WWII. She said that British planes would fly over Denmark to Germany and back during the night, and in the morning, there would be “silver streamers” out in the fields that were dropped by these planes. She said all the kids would play with them, turn them into crafts, etc. What were these?
Reflective chaff, codenamed "Window" by the British. The RAF bombed at night so the German defences were heavily dependant on radar, both for aiming flak guns on the surface and for vectoring in interceptors. Window was a countermeasure to this, by dropping little slips of paper coated in aluminum that reflected radar it would clog the signal. Instead of to detecting individual aircraft and attacking them all the Germans saw on their screens was a big blob moving towards them.
The history of Window is rather interesting, both the British and the Germans developed it but held off on using it out of fear that the other would retaliate in kind. Once the British committed to it it worked spectacularly, although given enough time the Germans did develop ways to effectively combat it using searchlights and different radar frequencies.
Sources:
The strategic bombing of Germany, 1940-1945 by Alan J. Levine, 1992
Fire and Fury: the Allied Bombing of Germany 1942–1945 by Randall Hansen, 2008
Bomber Offensive by Arthur Harris, 1947