Longtime children’s singer-songwriter and author, Barry Louis Polisar performs at Mechanicsville Elementary in Maryland on Friday, April 10. This spring, Polisar marks 40 years in the kids’ music business. (Photo by Amanda Voisard)

Sit on the floor, the children were told: “Crisscross, applesauce.” Listening ears on, everyone. Hands in your lap! (Hands in your lap, Kimmy.)

For here on the stage of their very own Mechanicsville Elementary School was a man so renowned that educators call him months in advance, cash at the ready, asking him to please, please come to their school, or their community center, or to the Easter Egg Roll at the White House.

And Barry Louis Polisar arrives wherever they request, ready to bestow upon small humans the lessons he has been teaching for decades. He stood now before nearly 200 third-, fourth- and fifth-graders to impart one of his most popular messages: Underwear is everywhere!

It is a song, and it goes like this:

Underwear is everywhere

But mostly underneath!

Usually, you can’t see

What goes on beneath!

He got about that far before roughly a third of the kids totally lost their crisscross applesauce. They rolled around, clasping mouths to contain their giggles, looking to their teachers with shocked eyes: You are letting this old man sing to us about underwear?!?

Ragged clothes, evening gowns

Or the finest three-piece suit

Underwear is everywhere

There is no substitute!

The teachers are giggling too.

This is the magic of Barry Louis Polisar, one of the most famous musicians of the children’s music scene — which is to say, not that famous. But within the genre, he is Bob Dylan, Steve Martin and your grandpa rolled into one. As of this spring, Polisar has been singing, dancing and fart-noise-making for 40 years. At 60, he is possibly the hardest-working man on the elementary school circuit — though, thanks to his under-the-radar commercial success, he probably doesn’t need to work anymore.

His songs, written on a nylon-stringed guitar he bought for $45 in 1972, are naughty and innocent at the same time. He doesn’t croon sentiments like “I love you, you love me.” He howls, “Don’t cook your sister in a frying pan!” His alphabet song begins, “A is for armpit, acne and alchemy, au naturel and alcohol, albatross and atrophy!” His lyrics once got him banned from the entire Anne Arundel County school system for a few months in 1990. He is sometimes asked: Why not use his talents to sing about world peace? Or preach recycling? How can he be sure that when he sings “Don’t put your finger up your nose,” kids won’t get the idea to put their fingers up their noses?

“If I get it,” he says, “They’ll get it.” And sorry, but world peace just doesn’t get laughs.

Kindergartner Nathaniel Goodrich, middle and Head Start students Jamarah Gore, left, and Sincere Wade, right, laugh while listening to Barry Louis Polisar perform. (Photo by Amanda Voisard)

He tells a story before he starts singing about how the song used to be about pants — but since everyone laughs at the word “underwear,” and laughing is scientifically proven to bring oxygen to your brain and make you smarter, “underwear” is the better rhetorical choice. The third-graders nod.

“Anybody can make kids laugh saying underwear,” Polisar explains. “What I’m trying to do is explore the language, to tell them why it is funny. My songs aren’t always to teach a lesson. They talk about what life is really like.”

Underwear with lots of holes

Is a sorry, sorry sight!

Look around and try to see

Who\’s wearing theirs too tight!

On this sunny Friday in April, Polisar drove his electric-blue PT Cruiser an hour and a half from his home in Burtonsville, Md., to perform four 45-minute shows at two Southern Maryland elementary schools for about 1,200 kids. He wore a crinkled brown leather jacket on the drive down but changed into the black blazer he keeps in his back seat before buzzing into the Mechanicsville Elementary front office. He is as warm and enthusiastic with the secretaries and the teachers as he is with his four-foot fan base. His gap-toothed smile is almost as constant as the lines it has indented on his cheeks.

His only showbiz request: two “not cold” bottles of water. Cold water, he explains apologetically, freezes his vocal cords. Everything else he can manage. A school doesn’t have a working microphone? He’ll grab a backup from the car. A girl in the front row gets sick and has to leave in the middle of the show? No problem; he carries on singing, “I Don’t Wanna Go to School.” A strict teacher tries to stop the kids from clapping along to “All I Want Is You”? He stomps his foot and nods his head to the beat to keep them going.

If that latter title rings a cheery, folksy bell in your mind, you might recognize it from the opening credits of “Juno,” the 2007 best-picture Oscar nominee. “All I Want Is You” is the reason Barry Louis Polisar wouldn’t have to be grade-school-road-tripping Barry Louis Polisar anymore, if he didn’t want to be.

All I want is you, will you be my bride?

Take me by the hand and stand by my side.

All I want is you, will you stay with me?

Hold in your arms and sway me like the sea.

Before he became a father of twins who inspired songs about diaper rash, before he met his wife, Roni, on his last day of class at the University of Maryland, Polisar was a 20-year-old undergrad who had begun to play guitar because he thought a friend’s older brothers looked cool playing guitar.

When a teacher learned that the young education major played music, she asked him to come to her school to perform. He wrote his first children’s song for the occasion, “I’m a Three-Toed, Triple-Eyed, Double-Jointed Dinosaur.” At the end of the show, another teacher scolded the kids for being too loud. So Polisar went home and wrote a song about a mean teacher. Then teachers at other schools heard about the mean teacher song and asked him to come sing it. Then he wrote some more songs. Then more schools called. And so it went, until he had enough music to make two albums by the time he graduated and enough gigs to forget about teaching and try music for a living.

On his second album, 1977’s “My Brother Thinks He’s a Banana,” he included “All I Want is You,” a harmonica-flavored tune with a bouncy melody irresistible to kids but lyrics that hint at grown-up infatuation.

If I was a flower growing wild and free

All I’d want is you to be my sweet honey bee

And if I was a tree growing tall and green

All I’d want is you to shade me and be my leaves

The song went mostly unnoticed, rarely gracing his set lists. Then in 2007, Polisar got a call from Jason Reitman. The 30-ish film director, then best known for “Thank You For Smoking,” had been browsing iTunes in search of songs that would set the mood for his next quirky dramatic comedy.

“Juno” told the story of a pregnant teenager in suburban Minnesota. Reitman said he fell for the script because of the emotional journey of a girl growing up too fast. Juno drinks Sunny D on the way to her pregnancy test. She tries to find cool adoptive parents for her baby, only to find herself disappointed by a would-be dad who’s not ready to grow up himself.

For Reitman, “All I Want Is You” represented Juno’s transition. “That’s the most beautiful thing about Barry’s music,” he said. “It seems to cohabitate both worlds, being an adult and being a child.”

Barry Louis Polisar has written more than 160 songs and authored 14 children’s books. Here he signs “The Snake Who Was Afraid of People.” (Photos by Amanda Voisard)

That balance is the quality that made Polisar’s music beloved — and now, lucrative as well. After “Juno,” “All I Want Is You” became a hit in a dozen countries, appearing in ads for Honda, Del Monte Green Beans, Vick’s, Coca-Cola and Ikea. The checks ranged from about $2,000 to $100,000 — more than Polisar could make in a year of $500-a-pop elementary school shows.

Advertisers like the song for the same reason parents and kids have loved tunes like “My Brother Thinks He’s a Banana” for decades.

“You can’t do something that appeals to just kids, because then you’re Barney, and the grown-ups are going to hate you,” says Kathy O’Connell, a radio personality who has hosted kids programs for 32 years. “If you can appeal to the kids and the parents who are trapped in the car listening, too, that’s when you have a repeat audience.”

Polisar’s biggest fans, perhaps, are children on the verge of becoming grown-ups, and grown-ups who don’t fully want to leave childhood behind. He can relate to both.

His parents divorced when he was 6. His mother remarried soon after to a man he describes as “detached” and had three more children. The youngest was a full 10 years younger than Polisar, and their quirks — a loathing of asparagus, for example — inspired many of his songs.

“I was old enough to watch the foibles and follies of my parents raising the young kids, but not old enough that I had to be the one to discipline them either,” he said. “I was just there, observing them. I think I felt out of place.”

He spent a lot of weekends with his grandparents a few miles away. They would discuss the civil rights movement and watch “Meet the Press” or old Westerns together. His grandfather died when he was 16, so in college he moved in with his grandmother to help take care of her in her last few months of life.

It was during his grandmother’s final year that he started singing for kids. She would ask him to recite his songs for her nurses, and she laughed at the punch lines every time.

“Maybe,” Polisar’s wife, Roni, says now, “that kind of experience gives you a bit of feeling of early independence in life, when you come to realize that somebody is leaning on you.”

At his Mechanicsville appearance, Polisar didn’t play his classic, “He Eats Asparagus, Why Can’t You Be That Way?” — but one little girl raised her hand with a question about it at the end of the show.

“Do you like asparagus now?” she asked.

On the sidelines of the gym, two women smiled at the question. Jennifer McGraw, 60, used to play Barry Louis Polisar songs for her daughter, Jessie Williams, now 40 and sitting beside her. Williams’s 9-year-old daughter Alex was sitting with the other kids in the audience. Of the three, only the grandmother likes asparagus.

Jessie Williams, 40, left, and her mother, Jennifer McGraw, 67, right, watch Barry Louis Polisar perform at Jessie’s daughter Alex’s school. (Photo by Amanda Voisard)

Barry Louis Polisar chats with multi-generational fans, Jessie Williams, her daughter, Alex Williams, and mother Jennifer McGraw. (Photo by Amanda Voisard)

“You know, I still don’t like asparagus,” Polisar replied, “But my daughter made asparagus soup and it’s one of my favorite foods, so figure that! In fact. . .”

He picked up his guitar for one last important message:

As I get older I’m beginning to see

Changes that are happenin’ to me.

I like to sit with girls and I like to ride the bus

But I still don’t like asparagus!

They get it. And they giggle.