Whoever thinks coloring books are just for kids has obviously never met Amazon’s customers.

In its annual post-holiday press release, chock-full of stats on what customers bought for and around the holidays, Amazon AMZN revealed (or reinforced) that we’re all still children at heart. An adult coloring book with mandala, paisley, nature and animal designs was among the top three best selling books this season, the company said.

Moreover, eight of the 20 currently top selling books on its website, according to the Wall Street Journal.

At first glance, this probably seems like yet another dumb fad for adults with too much time on their hands. But adult coloring books have taken off thanks to the belief that they helpful for relaxation.

Proponents argue that the focus required for coloring can help soothe people (think of how much more manageable kids become when you hand them coloring supplies at a restaurant). But not everyone is convinced.

Art therapist and psychotherapist Cathy Malchiodi has written critically about the claim that coloring is good therapy. In her opinion, coloring printed designs is not the same as creating art from scratch, an activity she says has actually been shown to be effective therapy.

But at least this holiday craze hasn’t gotten banned from airlines, Amazon, and New York City. For more on all the surprising or dull things Amazon customers bought, watched, and listened to, check out the full press release.