Every first Saturday of April, strangers show up to public squares and parks in cities across the world carrying pillows. No agenda, no ticket, no reason other than the one that gets forgotten quickest in adult life – to play.

The Urban Playground Movement started it in 2008 with a simple premise: public space belongs to people, and people need more from it than commutes and commerce. The gesture is small and the rules are simple, but what it points to isn't.

Reclaiming urban space for something genuinely useless is, perhaps, a political act. It asks who the city is for and answers it by filling the space with laughter and flying feathers. Studies on play in adult life consistently link it to reduced stress, stronger social bonds, and a greater sense of belonging – outcomes that are harder to manufacture than they are to accidentally stumble into during a pillow fight.

Here are shots of people from all walks of life enjoying a good fight; with the gentlest of weapons.