Glance at the stairs above.

Find the base…rather, spot the top. Upon closer examination, you’ll realize that there is no beginning or end. There’s no way that’s feasible, right?

These familiar steps, called the Penrose stairs, are a type of “impossible object”—a construction that could not exist in reality even though its individual pieces look totally valid, says Erez Freud, a cognitive neuroscientist at York University in Toronto. The paradoxical item takes form when the brain attempts to turn a 2D image into a 3D object.

From years of trusted experience, our noggins assume lines are always straight and corners precisely 90 degrees. But those facts can’t be true and still create this eternal four-way staircase. You can’t walk upstairs forever, especially in a loop. Normally, two regions in the brain’s visual cortex—the ventral visual pathway and the dorsal visual pathway—communicate to identify objects and place them in space. With impossible objects like the stairs above, those two pathways in your gray matter fire back and forth more than usual, trying to come to a conclusion. But they can’t, resulting in an uneasy feeling.

The key to this scene is perspective—it works only in 2D. If you saw these steps in real life, and viewed them from any other angle, you’d uncover what you always suspected: a gap between two sections. In other words, a staircase like any other.

This story appears in the Spring 2020, Origins issue of Popular Science.