This story has been brought to you by the letter S and the numbers 15 and 40. (Or, as the Count might say in his adorable Transylvanian accent, "fivteen and forrrty-HA, HA, HA!") The S, as anyone who has ever watched television can deduce by now, stands for Sesame Street. The 40 is almost as easy: this year marks the 40th anniversary of sunny days, friendly neighbors and the fuzzy creatures who live on that street where the air is sweet. If you haven't watched recently with your children or grandchildren, you'll be relieved to know that impending middle age hasn't wrinkled Sesame Street all that much. Big Bird still waddles, Cookie Monster still goes on his sugar binges and Ernie still wakes up Bert at all hours with questions (none of them, mercifully, about the nature of their relationship). In a world where cultural touchstones are dropping faster than the Mets in September-sorry, Guiding Light fans-the endurance of Sesame Street is nothing short of a miracle.

Which brings us to that second number of the day: 15. That, shockingly, is where Nielsen says Sesame Street ranks among the top children's shows on the air. Some months, it does even worse. Ask a preschooler who her favorite TV character is, and chances are she'll say Dora, Curious George or, heaven help us, SpongeBob. We know it doesn't seem nice to point out that the granddaddy of children's television is regularly beaten up by a girl who talks to her backpack, but these are desperate times. The Children's Television Workshop (now called Sesame Workshop) produces only 26 episodes a year now, down from a high of 130. The workshop itself recently announced it was laying off 20 percent of its staff as the recession continues to take a toll on nonprofit arts organizations. But Sesame Street is no ordinary nonprofit. It is, arguably, the most important children's program in the history of television. No show has affected the way we think about education, parenting, childhood development and cultural diversity, both in the United States and abroad, more than Big Bird and friends. You might even say that Sesame Street changed the world, one letter at a time. Don't believe us? Then let's imagine where we'd be if Sesame Street never existed.

For one thing, television itself might be a "vast wasteland." That was the phrase FCC chairman Newton Minow used to describe the TV landscape in 1961, and children's TV was hardly exempt. As recounted in Street Gang, a new book by TV journalist Michael Davis, the show came about after Lloyd Morrisett, an experimental psychologist, walked into his living room and found his 3-year-old daughter mesmerized by the TV test pattern. He told that story at a dinner party several weeks later and wondered aloud if children might be able to learn something from the boob tube. It seems like a crazy question in our Baby Einstein world, but back then, to paraphrase Donald Rumsfeld, we didn't know what we didn't know. When Sesame Street arrived, scientists were just discovering that our brains were not fully formed at birth and could be affected by early experiences. Head Start began in 1965, in part, out of that revelation. "Educators were virtually ignoring the intellect of preschool children," says Joan Ganz Cooney, who threw that dinner party and has been the show's visionary since the beginning. Children would eat up the ABCs before kindergarten, Cooney believed, especially if a wacky puppet ate up alphabet-shaped cookies along with them. The Department of Education was skeptical. Captain Kangaroo and Mister Rogers, though age-appropriate, had not become must-see TV; Bozo and Romper Room (which ended each show with the hostess pretending she could see children at home through a magic mirror that was obviously fake) presented dumbed-down fun. But the government agreed to contribute half of the original $8 million budget to launch Sesame Street. "It was a speculative leap," Morrisett says.

The results were pretty immediate. The first season in 1969 set out to teach children to count from one to 10, but it became clear that kids as young as 2 could make it to 20. (The show now hits 100, counting by tens.) That rookie year also yielded three Emmys, a Peabody Award, a front-page rave from The New York Times and one especially noteworthy piece of fan mail: "The many children and families now benefiting from 'Sesame Street' are participants in one of the most promising experiments in the history of that medium. The Children's Television Workshop certainly deserves the high praise it has been getting from young and old alike in every corner of the nation. This administration is enthusiastically committed to opening up opportunities for every youngster, particularly during his first five years of life, and is pleased to be among the sponsors of your distinguished program. Sincerely, Richard Nixon."

The most impressive feedback, however, came from the kids themselves-or at least from their test scores. No show to this day has probed its effects on kids as thoroughly as Sesame Street, which plans to spend more than $770,000 in 2009 on its department of education and research. When people think of Sesame Street as the essence of educational television, what they don't realize is how much the show has educated the educators. "Before Sesame Street, kindergartens taught very little," says Cooney, "and suddenly masses of children were coming in knowing letters and numbers." Independent research found that children who regularly watch Sesame Street gained more than nonviewers on tests of letter and number recognition, vocabulary and early math skills. One study, in 2001, revealed that the show's positive effects on reading and achievement lasted through high school. "It totally changed parental thinking about television," says Daniel Anderson, a psychologist at the University of Massachusetts.

But the show was never just about improving test scores. Perhaps the most radical part of the Sesame DNA has always been its social activism. From the start, Sesame targeted lower-income, urban kids-the ones who lived on streets with garbage cans sitting in front of their rowhouse apartments. The show arrived on the heels of riots in Washington, Baltimore, Cleveland and Chicago, and the assassination of Martin Luther King Jr. Chester Pierce, a Harvard professor who founded the Black Psychiatrists of America, was one of the show's original advisers, and he was acutely aware of the racism his 3-year-old daughter would face in that hostile time. "It was intentional from the beginning to show different races living together," says David Kleeman, executive director of the American Center for Children and Media. "They were very conscious of the modeling that kids and parents would take away from that."

In 1969, that was still a radical notion in some corners of the country. Here was a TV show putting African-Americans on a level playing field with white characters, showing them not as servants or entertainers, but as equals. (Though it should be noted that when the show premiered, some African-Americans took offense to Oscar the Grouch, who accepts his poverty rather than fighting against it, as a demeaning stand-in for inner-city blacks.) An integrated program aimed at impressionable children was too much for the good people of Mississippi. The state's commission for educational television banned the show in May 1970. Cooney called it "a tragedy for both the white and black children of Mississippi," and news reports saw her outrage and raised it. The state finally reversed itself, 22 days later. When you think about what the world might have looked like without Sesame, you can't dismiss the impact of putting Gordon and Susan into America's living rooms. Is it too much of a stretch to claim that the man in the White House might not be there without Sesame Street? "I like to think," Cooney says, "that we had something to do with Obama's election."

The show's impact has been as profound overseas. Sesame Street is now exported to 16 countries and regions-places such as the Palestinian territories, Kosovo and Bangladesh, where the message of tolerance can be in short supply. In South Africa, where as recently as 2008 the president insisted that HIV does not cause AIDS, the show features a ginger-colored, HIV-positive Muppet. The South African Sesame is also now produced in 12 of the country's official languages.

The show's we-are-the-world agenda doesn't always produce friendly neighbors. In 1998, a Middle East version was launched, co-produced by Israelis and Palestinians. The Israeli and Palestinian Muppets lived on different streets, but they would sometimes visit each other to play. Israeli Muppets could appear in Palestinian territory, but not without being invited. But the intifada made the notion of coexistence and cooperation politically untenable and it was canceled. The show returned in 2006, but now there are separate versions produced for Israel and the Palestinian territories. The Palestinian one no longer features Jews at all.

The tough topics aren't only political. Following the attacks of 9/11, the 33rd-season premiere found Elmo struggling to deal with his fear after he sees a grease fire break out at a lunch counter. He's reassured after he visits with real-life firefighters in Harlem. With that storyline, Sesame Street did more to acknowledge its audience's unsettled feelings than many adult shows did, even some set in Manhattan, including Friends and Sex and the City. In 1982, Will Lee, the man who played Mr. Hooper, died suddenly of a heart attack. The show decided to tackle the issue of death with an episode on Big Bird's distress and confusion over losing his friend. Children with illnesses and conditions such as Down syndrome are also regularly included. "For many children, the first place they may see a ballet may be on Sesame Street," said Rosemarie Truglio, vice president of education and research for Sesame Workshop, in a book about the show. "Moreover, it may be the only place where they see a ballet performed by a girl in a wheelchair."

Not everyone thinks that Sesame Street is doing right by kids. Latino groups have criticized it for not having a Hispanic character in its early years. The show only introduced a major female Muppet in 1992. (Prairie Dawn was too annoying to count as a role model.) It has also been criticized by Ralph Nader and the Campaign for a Commercial-Free Childhood for selling out its characters in too many licensing deals. Some of its interactive software products have been panned by Children's Technology Review.

There is no question that Sesame has provoked some critics to chastise it for getting a little too attached to the letters P and C. After the show launched an obesity-awareness campaign called Healthy Habits for Life, one particular Muppet needed to get with the program. So in 2005, Cookie Monster began to sing about cookies being "sometimes" food. Parents, some of whom wrongly believed that Cookie was going to become a health-food nut, started a preschool food fight. It turns out that Cookie still eats cookies in his typically frenzied fashion. "But the lesson was, this show is important," says executive producer Carol-Lynn Parente. "Don't mess with it."

That's impossible, of course. As Nicole Kidman might say about Botox, no 40-year-old looks young without a few touch-ups. (Cosmetic case in point: in the first season, Oscar was a particularly unattractive shade of orange.) Sesame Workshop is focusing a lot of energy on the digital universe. It recently launched a new Web site featuring a huge library of free video clips, both recent ones and classics. It also offers a series of podcasts that parentscan download to their phones to show their kids later, like when they're stuck in a long line at the grocery store. So in that sense, Sesame Street is no longer changing the world as much as trying to keep up with the world's changes. "We need to continuously reinvent or experiment," says CEO Gary Knell, "or else we are going to be dead."

Could that really happen-could Big Bird follow Mr. Hooper into the big playground in the sky? Maybe it's wrong to even worry about that. The granddaddy of them all doesn't have to survive for the breed to prosper; if that were true, people would still be driving Edsels. Children's programs are in more places than ever. But only a tiny handful, such as Blue's Clues or the new PBS show Super Why!, make any real attempt to conduct research like Sesame Workshop, not to mention influence the way the world thinks. If we agree that Sesame Street has changed our society, and many others, for the better, if we agree that we still need messages of open-mindedness and if we agree that it is still rare to find an educational television show that parents and children can enjoy watching together, then we have to hope that our furry gang will live on to greet the next generation of children. Can you tell me how to get to Sesame Street? Of course. The more important question now is: can you tell me if Sesame Street will continue to get to us?