Take a cloud, put it indoors, then photograph it. If it sounds simple enough, for Dutch artist Berndnaut Smilde, this is his lifetime's work - and it can take days to achieve the right result.
Smilde uses water vapour to create small clouds no more than six feet across in spaces as diverse as abandoned warehouses and grand houses with chandeliers. There's the perfect lighting and positioning to consider.
The cloud might last only a few seconds. Smilde employs a variety of photographers, usually with architectural backgrounds, to capture the exact moment he's looking for. That could take up to 100 clouds to realise.
The results are unlikely and disconcerting. What is that cloud doing in a semi-desert, hovering around graffiti or beneath those chandeliers?
"I see them as temporary sculptures of almost nothing," Smilde told 'Wired' magazine, "the edge of materiality."
His words echo those of the great Joni Mitchell from the 1960s when she sang, "It's cloud's illusions I recall/I really don't know clouds at all." Smilde's work is ethereal and therein lies its beauty. We'll never look at the sky in quite the same way again.