Sometimes the simplest illusions are the best. A simple trick makes a sketch of a cube appear to float and twist in the middle of a sketch pad.

This optical illusion is completely inexplicable when you first look at it, and then utterly comprehensible the moment the person in the video shows the trick. At first, a person simply holds a sketch pad. On the pad is a simple drawing of a box. When the person moves the sketch pad in a simple circle, the box dances on the pad, moving around fluidly and easily. How is that possible? In a movie it might be animation or CGI.

In life, an optical illusion is best because the viewer's brain is doing most of the work. The object on the pad looks at first glance like a drawn cube, letting the watcher assume a convex shape. When one set of lines on a cube shorten, the viewer assumes that that side has been turned away so less of the line is visible. When lines lengthen, the viewer assumes that that side is being turned towards them.

It's only when the entire pad is turned — cluing the watcher in that the entire shape was concave — that the optical illusion makes sense. It makes sense so much sense that you can't entirely re-capture the effect of the illusion for a while, after it's transformed back. This shows us that our eyes take in lines, but our brains construct shapes from them, and sometimes we construct them wrong.

Via Youtube.