The two best-selling books on Amazon right now are coloring books for adults.

Both “Secret Garden” and “Enchanted Forest” feature detailed black-and-white drawings of the flora and fauna that surround illustrator Johanna Basford’s Scottish home, and both are currently out of stock.

adult coloring books on Amazon“Secret Garden” has sold nearly 1.5 million copies since its debut, and “Enchanted Forest” has sold 226,000 copies since it was released in February.

Fans of the intricately illustrated books include celebrities like actress Zooey Deschanel, who recently shared her coloring masterpiece with her nearly 6 million Facebook fans.

And when NPR asked its followers if they were adults who liked to color, the response was overwhelming, with many people saying, “I thought I was alone.”

What’s got so many grown-ups engaged in an activity typically associated with children? It could simply be that adults are looking to get in touch with their inner child.

In the past few years, adult preschools and summer camps have emerged — complete with arts and crafts and campfire sing-alongs — and they’ve received rave reviews from participants.

Plus, marketers know you can’t underestimate the appeal of nostalgia. It’s why Buzzfeed has its "Rewind" section devoted entirely to nostalgic content.

“Bringing back the good memories of the past holds a certain comfort for a customer,” writes Nicole Kruzel on LonelyBrand. "In short, the explanation for the popularity of nostalgic content is that it makes us feel ... well, content."

Basford says there’s "definitely a nostalgic charm" to coloring her books, but she also thinks it’s simply a good way for busy adults to unwind and relax.

“It allows us to regress to a mindset when times were carefree and we didn’t have grown-up worries such as making rent payments or an annoying boss at work,” she told Slate. “It’s good to play. You rarely see a stressed out 7-year-old!"

Basford says she suspected there was a market for coloring books targeted at adults; however, she didn’t expect her creations to be quite so popular. So many people send her photos of their colorings that she’s created a gallery of her fans' work on her website.

“So many people have said to me that they used to do secret coloring in when their kids were in bed. Now it is socially acceptable, it’s a category of its own.”

Below, watch a time-lapse video of Basford creating an illustration for one of her books. She does all of her work by hand.

Laura Moss writes about a variety of topics with a focus on animals, science, language and culture. But she mostly writes about cats.

Why are adult coloring books so popular?
Our desire for nostalgia and relaxation may be why Amazon can't keep these books in stock.