IT IS the best-selling non-video game at Amazon.com. It also occupies the number two, three and four positions for its expansion packs. Elegant in its horrible simplicity, "Cards Against Humanity" (CAH) is the brainchild of a few friends who turned an informal pastime they developed first into a modestly successful Kickstarter project, and then a runaway mainstream commercial hit. The actress Anne Hathaway cited it (albeit erroneously as "Crimes Against Humanity") as her favourite in an Oscars red carpet interview. The firm orders new print runs by the literal boxcar. CAH involves black and white cards and any number of players. Those familiar with "Apples to Apples" will recognise the rules. Each player in turn becomes the judge for a round and reads aloud a black card that sports one or two blank spaces. It might read, "The way the world ends is not with a bang but with a ________." All other players, who have drawn and maintain ten white cards in their hands, hold possible answers. They range from amusing ("a centaur") to nonsensical ("YOU MUST CONSTRUCT ADDITIONAL PYLONS") to obscene or scatological ("genital piercings") to nearly unforgivable (and uncitable). All but the judge drop a white card face down into a pile. The judge reads through and picks the best answer by his lights. There is no defined way to win.

At the recent XOXO festival and conference in Portland, Oregon, CAH played a different game. Max Temkin, one of its inventors and its public promoter, attended last year's inaugural event. This time he was invited as a speaker. Rather than man a booth like last year, Mr Temkin opted instead for a trust exercise. He placed boxes full of the game and its add-ons on a table with an iPad configured as a Square cash register and a small sign that read, "Pay what you want." It showed the suggested retail price ($25 for the game and $10 for expansion packs). And it ended with, "Please do not steal our iPad."

The main floor of the YU, a contemporary-arts centre that was once a laundry, was unguarded. Any passer-by could enter. The table was in sight of other booths, but not under observation. The pile of cash grew over two days, and then the boxes, the table and the bills were gone. But thieves were not at work. The stock on hand had been sold. Mr Temkin tells Babbage that he had a backpack full of cash and no one absconded with the iPad. (No word on whether the bag raised eyebrows at airport security on his flight back to Chicago.)

The jape was in keeping with the CAH ethos. The game was released under a Creative Commons copyright license and remains free for download in a format suitable for printing and cutting from the firm's site. Mr Temkin says many people download the game to try it and then upgrade to its professionally printed version. The game's popularity has led to some variants that Mr Temkin says irk him, such as the parody version, "Crabs Adjust Humidity".

While he and his colleagues decline to discuss revenue, he recently explained in a blog that the firm had spent $150,000 for a big booth at the PAX Prime event in Seattle in August. Babbage was present and saw a stonking line of people buying the basic product and show-only specials, including the impractically large and innuendo-laden Bigger Blacker Box.

Babbage played his first game of CAH at the XOXO festival, bonding with new and old friends along with complete strangers, occasionally weeping with laughter. Mr Temkin sat in on a round with Babbage and his chums and "played" a card that he says he and the game creators had written but not yet released that involved insulting the thing in the blank. Several entries came in, but your correspondent had the winning hand by Mr Temkin's verdict. His white card read, "Cards Against Humanity".