“Yeah, but…” Fred Colon hesitated here. 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 a 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 weak even to him, so he added: “or a plinth(1). 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.”
“What about a potted plant?”
“That’s okay if it’s in an urn.”
“What about if it’s not got an urn or a plinth or a potted plant?” said Nobby.
“Have you got one in mind, Nobby?” said Colon suspiciously.
“Yes, The Goddess Anoia(2) Arising from the Cutlery,” said Nobby. “They’ve got it here. It was painted by a bloke with three i‘s in his name, which sounds pretty artistic to me.”
“The number of i‘s is important, Nobby,” said Sergeant Colon gravely, “but in these situations you have to ask yourself: ‘Where’s the cherub? If there’s a little pink fat kid holding a mirror or a fan or similar, then it’s still okay. Even if he’s grinning. Obviously you can’t get urns everywhere.”
— Terry Pratchett, Thud!, 2005
So, you see, it’s easy.