Stop the average American on the street and chances are you can get a correct answer to the musical question "Who can turn the world on with her smile?" No, not Sinead O'Connor. Mary Tyler Moore, whose comedy series was a consistent joy on CBS back in the days when there still was such a thing as joy at CBS.

Although it premiered on Sept. 19, 1970, CBS is calling this the show's 20th anniversary, mainly so as to lump it with tributes to "All in the Family" and "The Ed Sullivan Show" as part of the "Classic Weekend" that ends tonight with the "Mary Tyler Moore: The 20th Anniversary Show" at 9:30 on Channel 9 -- just after "Murphy Brown," the current sitcom that is fitfully successful at imitating Moore's.

Like the "All in the Family" retrospective, this one was planned for an hour but grew to 90 minutes.There are no bad clips included and countless good ones omitted. But rest assured that many of the best episodes are represented, including "Chuckles Bites the Dust," the bittersweet riot about laughing in the face of death.

Ironically, this was one of the few episodes of the series not directed by Jay Sandrich, the consummate pro whose amazing television career has stretched from "I Love Lucy" to "The Cosby Show." The Chuckles episode, about a clown who is shelled to death by an elephant while dressed as a peanut, was directed by Joan Darling and written by David Lloyd.

Watching this and other perfectly crafted gems, one is reminded not only of the show's high standards and high quality, but of how caught up we were in the lives of the characters, and how caught up they seemed to be in ours.

Of course it's a nostalgic as well as entertaining occasion, and the special is laid out that way. Producer-director Jack Haley Jr., who did the "That's Entertainment!" films, reassembles the surviving cast members in a living room setting to watch their former lives pass by on a big Sony TV set -- chatting, chuckling and getting weepy at the appropriate moments.

Ted Knight, who played fatuous anchor Ted Baxter, the quintessential dunderhead, died in 1986, but he is well represented in clips, which include the daffy golden moment when he met his idol, Walter Cronkite. We also see Ted's marriage to the wispy yet flinty Georgette (Georgia Engel) and the birth of their baby, which reduced Ted to a state of stammering imbecility -- not that much of a change, actually.

That Moore was the boss as well as the star is established in the first half-hour of clips, which center on her in her role as Mary Richards and on Richards's relationship with news director Lou Grant, played by Edward Asner. Today, Asner has grown wider and whiter-haired and looks like he should be cast as a Soviet party boss; then, they were the perfect mismatched pair.

The good vibrations began in the first episode, when Mary Richards, pulling her life together after having been dumped by a boyfriend, arrived in Minneapolis and applied for a job at WJM-TV, Channel 12. This engendered what became for television a historic exchange.

Lou: "You know what? You've got spunk."

Mary: "Well, yes -- "

Lou: "I hate spunk."

Others on the couch and in the excerpts include Valerie Harper, who played Rhoda Morgenstern; Gavin MacLeod, who played Murray Slaughter; Cloris Leachman, who played Phyllis Lindstrom; and Betty White, who did not join the series until its fourth season (most people think she was always there) as the predatory happy homemaker, Sue Ann Nivens.

Yes, we get to see again the time that Murray dropped Sue Ann into a wedding cake.

Yes, we get to see again the time Mary and Lou went on a date.

Yes, we get to see again, at some length, the tearful but funny farewells from the last episode, when a new owner had bought WJM and fired everyone on the staff except for the incompetent Baxter. He saluted his fallen co-workers on the air by reciting lyrics from "It's a Long Way to Tipperary."

The joke was that the song was absurdly inappropriate. But it does contain a reference to "the sweetest gal I know," and Tipperary does resemble the WJM newsroom in that many people could say of it, "My heart's right there."

Backstage stuff is in short supply on the special; nor are there any of the endearing outtakes that were shown at the cast's last wrap party and have been circulating for years on the underground circuit. But for those who want to know more about this show and thus about the very collaborative art of episodic television, the "Moore" story is nicely told in a 1989 book by Robert S. Alley and Irby B. Brown called, after a line from the title tune, "Love Is All Around."

The authors point out that many CBS executives hated the concept from Word One and that even savvy Fred Silverman, then a CBS programmer, had misgivings, insisting that Mary Richards get married sometime during the first season. It was thought the audience would not accept a series about a single working woman. Therein lay the breakthrough.

Originally, Mary was to be divorced, but it was feared that would make her unsympathetic (even now, most of the innumerable single parents in TV sitcoms are widows or widowers, despite the country's 50 percent divorce rate). According to an early plan, Murray was going to be gay, with a pair of ice skates in his desk drawer. Somewhere, that notion was dropped.

It's instructive, too, to recall critical reaction to the series premiere. The New York Times called the show "preposterous," and Time magazine labeled it "a disaster." Time has hardly ever been right about a TV series since. While the show has achieved near mythic status now, the CBS schedule was so strong in the '70s that it had plenty of company, especially on Saturday nights.

In the 1973-74 season, for instance, this was the CBS Saturday night lineup: "All in the Family," "M*A*S*H," "The Mary Tyler Moore Show," "The Bob Newhart Show" and "The Carol Burnett Show." Incredible! Networks today are lucky to have that many good shows in their entire week's offerings.

Maybe ailing CBS should buy back the reruns for all these shows and put that Saturday night lineup back together again. There reportedly has been talk of rerunning "All in the Family" in prime time, as one way of recapturing past luster.

Near the end of tonight's too-short special, Moore, wearing the obligatory unflattering dress, mentions some of the "creative people" behind the scenes who contributed to her show's success, among them James L. Brooks, now co-producer of "The Simpsons." Alas, she does not mention Grant A. Tinker, who ran MTM Enterprises during the glory days of this program, and of "Hill Street Blues" and "Lou Grant" too, and is Moore's ex-husband.

That omission is the only sour note in one sublimely sweet show.