Real talk: how does Santa get down the chimney? Mac Barnett has been wondering since he was a little kid.

"I lived in an apartment that didn't have a chimney," the author says. "So I talked a lot with my mom about how Santa was going to get into our apartment and give us gifts."

There are, of course, a lot of possible solutions.

"Does he cinch up his belt? Or shrink himself down to the size of a mouse? Or stretch out like taffy and step in one leg at a time?" Barnett hypothesizes in his aptly titled new children's book, How Does Santa Go Down the Chimney?

"This felt like a chance to set up this problem and to propose a bunch of solutions," say Barnett, "and basically put Santa through the wringer in the pictures."

How Does Santa Go Down The Chimney? is illustrated by Barnett's frequent collaborator, Jon Klassen. The two have worked on the Caldecott Honor book Sam and Dave Dig a Hole, as well as The Wolf, the Duck, and the Mouse, and The Shapes Trilogy series. They are also friends — they met at a book party over a decade ago when they were both at the beginning of their careers.

"I loved his work so much," says Barnett, "so I just, like, beelined for Jon as soon as he walked into the party."

"He's like, 'I heard you like Frog and Toad,'" remembers Klassen, "and I just said, 'I love Frog and Toad.'" They talked about Frog and Toad, a series of children's books about two amphibious friends written and illustrated by Arnold Lobel, for the rest of the party — and most of the last 13 years.

Now, one thing to get out of the way early on: Mac Barnett and Jon Klassen have no idea how Santa does what he does.

"We know a lot about what Santa gets up to," says Barnett. "We know where he lives, we know how he gets to the house — the flying reindeer. Where he works on his presents." But, as fans of the postal system will know, it's the last leg of the journey that is the most difficult.

"This is a classic last mile problem," Barnett, who is a fan of the postal system, says.

"It's strange that there's no definitive answer to this," adds Klassen, "considering this is a guy who does this every year."

Kids, they find, don't usually care so much about answers. So instead of answers, Barnett and Klassen offer an abundance of theories. Maybe the reindeer give Santa a little shove down the chimney? For apartments, maybe Santa uses the spare key and walks through the front door.

Jon Klassen says he illustrated this picture book with The Far Side comic strip by Gary Larson in mind. "Most of them, the moment that he chooses to draw is either before the thing that is about to happen or after it's happened," says Klassen. "It's never, like, the explosion. It's never the actual event."

A lot of the illustrations in How Does Santa Come Down The Chimney are Santa ... standing there. Staring at the chimney, scratching his head.

"Those moments of him just staring at his assignment were the funniest for me," says Klassen. It was also fun, he says, to draw Santa getting absolutely destroyed by his job. In this book, Santa gets flattened, squished, kicked, and even liquified. Luckily you can do almost anything to the big guy in the red hat with the white beard and still tell he's Santa.

"We can send him through the pipes and out your faucet and you're still going to recognize that it's him," says Barnett. "Poor guy."

Jon Klassen says he tried to draw this children's story with kind of an edge. If you think about it, a book about Santa creeping into your home in the middle of the night is not not a little scary.

"If it was too cozy, you know, all the way through, it might not be quite as funny," explains Klassen.

So he illustrated the book with a lot of big, black sky for the exterior images. The story starts with Santa outside and gradually moves inside as Santa contemplates barking dogs and plates of food. Inside, there are a lot of warm, dimly lit interior spaces.

"I had a paper route when I was growing up," says Klassen. "You end up standing in a lot of entryways in the snowy times waiting to get paid or waiting to deliver the paper properly or something. And in Niagara Falls, where this was, it was just a lot of old, tiled, warm entryways."

And, of course, the edge: a slightly bemused looking Santa "creeping under your door as a flattened envelope or something," Klassen adds. The Envelope Method is Mac Barnett's personal favorite solution.

"If you've got a mail slot, I bet Santa folds up like a letter and has a reindeer pop him through," writes Barnett. "And I bet the stamp on Santa is one of those Santa stamps too."

"I think that just appeals to my tendency toward endlessly recursive images," he explains.

Santa, meanwhile, wears an expression that suggests he's resigned himself to participating in this story.

"His attitude is a little bit bored, I think," says Jon Klassen. Poor Santa keeps running into problems trying to get into these houses — at one point his author says, "Maybe Santa can turn into fire!" Santa seems to like this idea.

Then, quickly, "But probably not."

As Santa ties himself into knots trying to humor the author and the illustrator, you get the sense he's just biding his time until he can actually do it the real, super secret correct way.

"I thought of it as a joke book," says Jon Klassen to Mac Barnett. And, don't get him wrong, the book is very funny. "But... you land it in a way that's very gentle and soft and actually very sweet about Christmas."

Barnett says he wanted the ending to this book to capture how magical Santa really is. "I think we don't want to know how he does it," he says. "The mystery, the wonder, the impossibility is the point. It's beautiful to kind of face the impossible that way."