Adult coloring has become a thing — and with good reason. Not only is the practice beneficial for people with specific conditions, like PTSD and those suffering the psychological side effects of cancer diagnosis and treatment, it’s a good stress reliever for the general population.

It’s also just plain fun.

So what makes coloring such an effective form of art therapy?

“The action involves both logic, by which we color forms, and creativity, when mixing and matching colors. This incorporates the areas of the cerebral cortex involved in vision and fine motor skills (coordination necessary to make small, precise movements),” psychologist Gloria Martinez Ayala told the Huffington Post. “The relaxation that it provides lowers the activity of the amygdala, a basic part of our brain involved in controlling emotion that is affected by stress.”

Put another way: When we focus on coloring, it blocks our brains from focusing on our troubles.

“Because it’s a centering activity, the amygdala, which is the part of the brain that is involved with our fear response, actually gets a bit, a little bit of a rest,” psychologist Dr. Ben Michaelis told Fox News. “It ultimately has a really calming effect over time.”

Another appealing aspect of adult coloring? The low barrier to entry. All you need is a coloring book and some crayons or colored pencils — and the “But-I-don’t-know-how!” intimidation factor is non-existent.

Heidi Johnson, who participates in a monthly coloring club in Minnesota, told Fox: “You don’t have to be an expert at the DIY project, or a painter or an artist. You can just pick something you like that speaks to you and color it however you want it.”

Yet coloring has a similar psychological effect as those arts and crafts that require more skill building. The repetition and predictable outcome — much like when a person knits or embroiders — is soothing, almost like meditation, neuropsychologist Stan Rodski told the Australian Broadcasting Corporation.

The results were even measurable, with scans showing that coloring will slow heart rate and change brainwaves.

The only tough part is choosing the right coloring book. In the wake of all this pro-coloring news, the adult coloring book market is booming.