The popularity of coloring books for adults doesn't seem to be waning. I thought this would be a passing fad, but it looks like I'm wrong. The number and variety of people who have taken up the hobby shows no sign of slacking.

Coloring books now have their own Amazon category page with versions for plenty for proclivities from Bible-themed to books covering unicorns, butterflies or owls, cats and dogs, dream scenarios, positive affirmations, several specifically for stress reduction, even a "Game of Thrones" coloring book.

There are plenty of theories behind why a traditional time-filler for children has become so popular, but it has at least something to do with stress relief, among other things. I asked my friends on Facebook why they loved it, and I got a surprising number of answers. I had no idea so many people I knew enjoyed coloring until I asked!

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MNN's own Robin Shreeves is a fan, and it's partially because of the ease with which she can pick up a coloring project: "I enjoy them because when I'm doing them, I don't think about anything but colors. They take me away from life's problems without a lot of effort. So do many other creative endeavors, but with the coloring book, I can do it for 10 minutes instead of the time it would take me to do some other things. I don't do it often, but I pick it up from time to time and get lost for a bit," Shreeves wrote.

"Coloring fulfills a creative urge and is also soothing and calming," wrote Jan Hornbeck Chapman, an Ohio-based youth librarian in her 60s.

Sophie Hessekiel, a college student at Vassar in Poughkeepsie, New York, echoes Chapman, even though she's at a completely different stage of life. "Coloring lets you think about nothing for a little while, and the feel of a marker on paper is very soothing," she wrote.

But Hessekiel also points out that coloring isn't the singular activity you might assume it is: "It can even be social — sometimes I have my friends over and we all work on one page at the same time," Hessekiel added. (Sounds fun, doesn't it?)

Secret Garden coloring book illustrationA spread from Johanna Basford's adult coloring book, 'Secret Garden.' (Photo courtesy of Laurence King Publishing)

Anne Hogue from Corvallis, Oregon, started using adult coloring books as a way to meditate. Hogue, who has dealt with chronic fatigue syndrome for more than 35 years, said the activity was relaxing and therapeutic. "You turn on the news and see terrorism and violence and terrible things, and then you get out the coloring book and it relaxes your mind," wrote Hogue.

Recently a family friend lost her husband and was struggling with the quiet house after work. Hogue sent her some coloring pages, and her friend reported that it was helping her through some difficult times.

For some, coloring can even be sort of puzzle: "It's more than just regressing back to what you enjoyed as a young child," points out Jennifer Veilleux, a photographer and geographer who specializes in international waterways. "Some artists even hide animals or symbols in the patterns that only become visible when you fill in the areas around them. You can channel your inner 6 year old, but now you know more about shading and texture," Veilleux writes.

All these thoughtful comments helped me realize why this activity is so beloved by so many — the ready-for-marker (or colored pencil) drawings are a blank canvas. This allows fans to create a world of their own — with a little guidance if they stay inside the lines.

It's reassuring, simple and has pretty results; there aren't that many things you can say that about these days.

Starre Vartan ( @ecochickie ) covers conscious consumption, health and science as she travels the world exploring new cultures and ideas.