You’re at a party at Mary’s, and the conversation is as sparkling as the champagne cocktails. Suddenly her new friend Mark makes a sexist, racist, or otherwise offensive joke, and the fun deflates like a whoopee cushion. What do you do?

“Chances are, all conversation will cease while those present pretend to laugh,” says Letitia Baldrige, author of Letitia Baldrige’s New Manners for New Times, A Complete Guide to Etiquette (Scribner, $24, www.amazon.com). But the last thing you should do is laugh to make Mark feel more comfortable. That’s just condoning the joke.

If the comment can actually do harm — if it’s maligning someone’s reputation, say — you should, of course, “step up to the plate; don’t just sit there,” says Baldrige. “But if the offender is just being tasteless and a boor, it’s punishment enough if you quickly change the subject. He’s certain to feel the sting from that. To be more direct — for example, saying ‘I find that joke incredibly tasteless’—would, in my opinion, be terribly grand and snooty sounding.” And dwelling on Mark’s offense will only lead to an early night for all.

Instead, Baldrige suggests piping up with something like “Michelle, tell us about your trip to Paris. Was it fabulous?” or introducing a topic from the news. Perhaps the obvious slight will embarrass Mark enough that he’ll think twice about telling a similar joke next time.