WHAT would a child-friendly city look like? One scenario goes like this: you wake up in the city one morning, there is no traffic, all you can hear are children playing and the occasional dog barking. All around you, muffling sound and covering dirt, is a magical material –- snow. It is malleable. It has endless possibilities. It turns hills into giant slides and you can build with it.

Snow days are, of course, temporary aberrations within urban life. As such, they are rapidly corrected by snowploughs, salt and gravel, or, in warmer countries, simply by the weather. Snow turns to slush, traffic returns, school resumes.

Another possibility was offered by the Dutch architect and playground designer Aldo van Eyck. Between 1947 and 1978, he designed and built around 700 playgrounds in Amsterdam. Sand, not snow, was his magical material.