The folks of the YouTube channel brusspup demonstrate the “Crazy Nuts” illusion originally created by magician Jerry Andrus.
This illusion exploits the way our brain constructs perception of depth and volume using cues from lines, lighted and shaded surfaces, and knowledge of the geometry of known objects. The effect is similar to that shown in the photograph below of a section of a building at the Glenstone Museum in Potomac, Maryland. The photograph shows two walls joining at a midline which is away from the observer with a section of blue sky visible on top. Viewed this way, the illumination seems to come from the left. Thus, the right wall is more in the light, and the left wall is more in the shadow due to the angle. However, the photograph can also be seen as the edge of a tower with a blue top where the two walls join at a midline which protrudes towards the observer. Viewed this way, the illumination seems to come from the right.
The Glenstone Museum photograph belongs to the author and can only be used with permission.