Turn your head from side to side for a 3D effect.

The red letters above may seem to hover above the blue background, especially when you move your head around. I’ve noticed the effect is much more pronounced when I’m wearing glasses. For some people the effect is reversed, with blue standing out and red receding. This phenomenon is known as chromostereopsis, and was noted by Goethe in his 1810 book Theory of Colors. It can produce striking effects.

Here’s what I think is so amazing about chromostereopsis: if you focus on an area of blue that is very close to the edge of a red area, and then slowly turn your head, you can actually see the blue pattern disappearing behind the red. It even turns kind of purple.

Chromostereopsis appears to have been in use as an artistic technique as early as the 16th century, when the stained glass piece above was created. The effect seems particularly vivid on computer and smartphone screens, which may be why it is enjoying more popularity recently, including its use in this 2015 movie poster.