NEW YORK (AP) — For opera fans, the sight of rattled tenor Roberto Alagna storming offstage after being booed at Milan's La Scala was stunning. But for those who don't follow opera, what was really stunning was the booing itself.We pay our money to hear a favourite singer, watch an actor, root for an athlete or a team.

In return, we expect something. If we don't get it, is booing justified? Or is it the very height of rudeness to hurl such a humiliation at someone who's summoned the courage to appear before thousands of people?

In sports, booing is part of the game. When even the home team is performing badly, be it on the baseball diamond or the football field or in the hockey arena, they can expect to hear about it.

"It's accepted and actually considered part of the environment," says David Cummings, senior deputy editor at ESPN The Magazine. Not that it doesn't rattle or even anguish the player in question. But even so, Cummings says, "athletes know it comes with the territory."

An exception is the manicured world of golf, where booing has no place. And in tennis, the occasional boo for a favourite's opponent — especially when the opponent is serving — will be greeted with a round of "shhhhhh" from other fans. People don't boo when their favourite is losing; they offer chants of encouragement.

Then there's opera. If you can't boo on the golf course or at Wimbledon, why can you boo in a gilded, chandeliered opera house? Of all entertainment forms, opera is the only one where booing has a long tradition, rooted in the intense emotion of the entire experience, say aficionados.

"It's a deep, deep passion," says Mary Lou Falcone, a public relations specialist in classical music. "With opera you have the meshing of the words and the music — the theatre of it is just something that people get into, and relate to."

Booing at operas is much rarer in the United States than in Europe, where it's in the fabric of the culture to express oneself loudly — either with boos or with bravos. Often the director of a new production, or the set and costume designer, will be booed for going against tradition. But singers are booed, too.

It's unfair, says Falcone, who represents American star Renee Fleming, among others. "Would that the booers would ever have the opportunity to be onstage, and to feel it once," says Falcone. "I don't think they'd ever boo again. It's an act of courage to go out there."

Opera fan Jeff Linder agrees, to a point. "I know they're putting their lives on the line every time they go out there," says Linder, of Great Neck, N.Y. "I count myself as more polite than the booers." However, he allows for the rare case where he might be inclined to boo. "If someone did a really, really bad job, and they kept doing it, I might," Linder says. "Just to say, 'Don't bring this guy back again."'

But it's all in the environment; Linder can't easily imagine booing at a Broadway show, for example.

Booing is pretty much unheard of at the theatre. What many have noticed is the reverse phenomenon: the ubiquitous standing ovation.

"It's almost de rigeur," says Alison Fraser, an actress who's been nominated twice for Tony awards. "There are a lot of really indiscriminate standing ovations going on." She estimates that it's been happening for the last couple of years.

What's behind it? The most popular theory is that people spend so much money on a ticket — as much as $350 for a "premium" seat — that they need to feel it was really, really worth it.

As Jed Bernstein, former head of the League of American Theaters and Producers, describes it: "I spent $100 so it must have been great. How do I know it was great? Because I'm standing here clapping."

But that's the cynical explanation, he says. He prefers the theory that people are responding to something vital: the special energy of a live entertainment experience, which is rarer and rarer these days.

"They are reacting to the uniqueness of that experience," says Bernstein, now president of Above the Title entertainment. "It's a subliminal energy transference."

And it seems that nowhere is that interactive energy more highly charged than at the opera, where fans pay much more than for a Broadway show, and pride themselves on encyclopaedic knowledge of the works and the singers.

John G. Miller attends twice a week during the Metropolitan Opera season in New York.

He recalls a recent highly anticipated performance of "La Boheme" with Placido Domingo — conducting this time, not singing.

Operagoers report he was clearly taken aback when he returned after an intermission and was booed from a corner of the balcony.

Was it someone with a private axe to grind, or a reaction to the performance? Miller thinks it was the conducting.

"It was the worst conducted Boheme I've seen," he said. "And I adore Boheme. I didn't boo, out of respect for Domingo's career. But I was tempted."

As it happens, Miller's a fan of the New York Knicks, too. And that's not a happy place to be right now.

The 9-and-15 Knicks are playing to less-than-capacity crowds at Madison Square Garden, where the fans vent their frustration on star guard Stephon Marbury and call for the firing of coach Isiah Thomas.

"Have you been to a Knicks game lately?" Miller asks. "Half the crowd is gone by the second period."

And the rest?

"They stay to boo," he says.