A pillow is a "combat tool" which seemed to be specially invented for women to fight with. It's soft, almost harmless and usually at hand. A pillow seems to be the only combat tool that women use much more often than men are. (Of course, if we don't consider the dangerous women's weapon - a rolling pin or a frying pan. )
A pillow fight is a truly folk pastime. There are many forms of pillow fights - from a fight with pillows in a bedroom to a real amusing sport, in which contests are well organized and contestants are supplied with special bags or real pillows.
"Fighters" in pillow fights act in various arenas (besides bedrooms) - on the ground, on the floor, on the small rafts. They compete standing on a log, sitting astride it, etc.
In organized pillow bouts, usually the one who first falls down on the ground or into water, usually is announced a loser. Such bouts are held outdoor, in rings or even in MMA cages.
Pillow fights above water pools are very popular on cruise ships and sea resorts
Well organized pillow fight championships are held with crowd of spectators. Girls seem to be especially attractive when fight with pillow!
Nowadays, mass pillow fights have become very popular in cities' streets and squares. Crowds on young girls and guys get together and instantly start fighting. The excitement of such an actione is increased by the fact that police can consider such an event illegal. 
Videoclips
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Pillow fight championship in a resort
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 Pillow fight in a cage. The girls started by the rules but then got excited and began ardent wrestling
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 Mass pillow fight in Zurich, April 04 2009
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