But why would it even be interesting—let alone soothing—that two random, unrelated items could be physically combined?

“I think it has to do with a new way of putting things together in a surprising, novel, and exciting way. An unexpected way,” says Johan Wagemans, a professor of experimental psychology at the University of Leuven in Belgium whose research focuses on perceptual organization. “Suddenly you see things in a different relationship and it challenges your expectations about how things relate to each other.”

The unusualness of the combination might grab the attention, but it’s also how extreme the coincidence of a perfect fit is. “If you look at it statistically, and in terms of how these objects are made, and how they came together, it’s almost too good to be coincidental,” Wagemans says. “If it would fit only 75 or 85 percent, it wouldn’t be fun.”

It’s the sort of little joy that can’t be forced, only discovered.

“I think in the times in which we live, we are so inundated with so many things, that to be able to bring order to things that don’t necessarily go together, to make them fit, provides some sort of comfort in a world where there’s all sorts of different things coming at you in all different ways,” says Gillian Roper, a psychologist based in Beltsville, Maryland.

In its bringing of order to randomness, “Things Fitting Perfectly Into Other Things” is a spiritual sibling to another popular Tumblr, “Things Organized Neatly,” which specializes in photos of objects arranged in patterns by size, color, type, or shape, laid out in tidy rows and columns.

Roper, who consults as a psychologist at nursing homes four days a week, has a side business as a professional organizer and is a member of the National Association of Professional Organizers.

“What I do is very intangible,” she says of her psychology work. “Part of the reason I enjoy organizing [is that] I enjoy the actual hands-on, being able to take a space that is total chaos and bring some order to it.” A physical counterpoint to a mental, abstract sort of labor.

Both Roper and Wagemans theorized that humans have an innate desire to put things in order. They both mentioned that people with autism have a tendency to line up and stack objects, saying that this may just be an extreme version of a normal human inclination.

The tactile component Roper talks about is part of it, Wagemans says, but also “the action component—making it fit, doing something to the world that makes the world more orderly. I think it is the pleasure of control.”

It seems likely that the pleasure of looking at these photos is a diluted version of the pleasure of doing the thing itself—be that organizing or finding a thing that fits perfectly into another thing. BuzzFeed, of course, as ever, is on it. Posts like “26 Pictures That Will Give You Some Peace For Once In Your Life,” “29 Photos That Will Make You Breathe Easy,” and “34 Photos That Will Satisfy All Perfectionists,” are compendiums of things fitting perfectly into other things, things organized neatly, and a couple of other categories I’ll call “things being removed smoothly” (an apple being peeled in a single spiral, for instance) and “things exhibiting unusually perfect shapes” (such as an impeccably spherical snowball). The jumbling together of these categories without distinguishing them suggests they evoke a similar sort of satisfaction in the viewer—judging by the millions of views on these posts.