In 1947, the architect Aldo van Eyck built his first playground in Amsterdam, on the Bertelmanplein. Many hundreds more followed, in a spatial experiment that has (positively) marked the childhood of an entire generation. Though largely disappeared, defunct and forgotten today, these playgrounds represent one of the most emblematic of architectural interventions in a pivotal time: the shift from the top down organization of space by modernist functionalist architects, towards a bottom-up architecture that literally aimed to give space to the imagination.