I’ve certainly never come across any irreversible mathematics involving sofas

He peered at Richard seriously. “Do you have a good sofa?” he enquired.

“Well, yes.” Richard laughed. He was cheered by the silliness of the question.

“Oh,” said Reg solemnly. “Well; I wish you’d tell me where you got it. I have endless trouble with them, quite endless. Never found a comfortable one in all my life. How do you find yours?” He encountered, with a slight air of surprise, a small silver tray he had left out with a decanter of port and three glasses.

“Well, it’s odd you should ask that,” said Richard. “I’ve never sat on it.”

“Very wise,” insisted Reg earnestly, “very, very wise.” He went through a palaver similar to his previous one with his coat and hat.

“Not that I wouldn’t like to,” said Richard. “It’s just that it’s stuck halfway up a long flight of stairs which leads up into my flat. As far as I can make it out, the delivery men got it part way up the stairs, got it stuck, turned it around any way they could, couldn’t get it any further, and then found, curiously enough, that they couldn’t get it back down again. Now, that should be impossible.”—Douglas Adams, Dirk Gently’s Holistic Detective Agency

Rosemary and I got a new sofa. Guess what.