Ever been browsing for books at Amazon and notice the “recommendations” area? I’m talking about that section on each page where it says “Customers who shopped for this item also shopped for these items.”

It’s a bit of artificial intelligence. Amazon keeps track of what books each customer is browsing, and uses “collaborative filtering” to automatically detect patterns. So if a bunch of people who browse The Nanny Diaries are also browsing, say, I Don’t Know How She Does It, then presto — Amazon’s artificial-intelligence agent will make the link between the two titles, and let you know.

But here’s where it gets fun. Recently, an Internet-security expert was looking at a Pat Robertson book when he looked down to see “Customers who shopped for this item also shopped for” … The Ultimate Guide to Anal Sex, by Bill Brent. I’m not kidding: the screenshot is here.

Pranksters, it seems, had hacked the system. It’s not hard to do. All you do is go to page for the Pat Robertson book, then click over to the new title you want to link it to. Do this a couple dozen times yourself, and get a few dozen friends to do the same. Soon, the filtering mechanism will sense the pattern, and bingo!

It’s a great lesson about machine intelligence. Corporations continually surround us with A.I. tools, and sort of hope we’ll just be in awe of them — and not take the time to find out what’s under the hood. Eventually, of course, we do, and we discover the paradox of A.I.: That the strongest-seeming artificial intelligence is usually based on very simple techniques. Faced with the spectacle of Christian anal sex, Amazon had to go in and manually break the link:

“It seemed to us that this is a rather curious juxtaposition of the two titles,” said Amazon spokeswoman Patty Smith.

The Robertson book now links to “clean underwear” and “ladybug rain boots,” which actually to my mind is far more alarming, but whatever.