There haven't been a lot of experiments on why we get random insights, but psychology does have one theory that describes a mental state that seems to foment these kinds of thoughts. It's called the default mode network.

"You become less aware of your environment and more aware of your internal thoughts," John Kounios, a psychologist who studies creativity and distraction at Drexel University in Philadelphia, told WIRED.

The common thing in these activities is they are physically or mentally active, but only mildly — they're also familiar or comfortable enough that we stay engaged but not bored, and last long enough to have an uninterrupted stream of thought.

Kounios explained that our brains typically catalog things by their context: Windows are parts of buildings, and the stars belong in the night sky. Ideas will always mingle to some degree, but when we’re focused on a specific task our thinking tends to be linear.

Kounios likes to explain it with a stack of bricks in your backyard. You walk by them every day with hardly a second thought, and if asked you'd describe them as a building material (maybe for that pizza oven you keep meaning to put together). But one day in the shower, you start thinking about your neighbor's walnut tree.

"Those nuts sure look tasty, and they’ve been falling in your yard."

You suddenly realize that you can smash those nuts open using the bricks in your backyard!

As far as Eureka moments go, using a brick as a nutcracker is pretty lame, but as an illustration of how the default mode network frees the things in your brain from external associations, it works wonderfully. As ideas become untethered, they are free to bump up against other ideas they've never had the chance to encounter, increasing the likelihood of a useful connection. And this guy seems to be aware of that!