He knew in his heart that spinning upside down around a pole wearing a costume you could floss with definitely was not Art, and being painted lying on a bed wearing nothing but a smile and small bunch of grapes was good solid Art, but putting your finger on why this was the case was a bit tricky.

'No urns,' he said at last.

'What urns?' said Nobby.

'Nude women are only Art if there's an urn in it,' said Fred Colon. This sounded a bit weak even to him, so he added, 'or a plinth. Both is best, o' course. It's a secret sign, see, that they put in to say that it's Art and okay to look at.'

~ Terry Pratchett, Thud!