If you look at the pulsating image below, you likely see the colored lines changing in length. But in reality, the lines never change—only the direction of the arrows do. Based on the Müller-Lyer optical illusion—in which lines appear shorter or longer based on what direction the arrows at their endpoints are facing—the animated illusion consists of 10 lines radiating out from a central point, with arrows set at the midpoint and endpoint of each line. Each line is half red and half blue.
As part of the animation, the arrows change direction, sometimes facing away from the center of the illustration, and sometimes facing toward it. The arrows of each line move in the opposite direction from one another, so that the middle arrow points toward the center when the arrow at the endpoint points outward, and vice versa.
The Müller-Lyer illusion shows that people perceive a line being longer if the arrow points away from its center, so when the arrows change direction on the animation, it looks like half of the line is getting longer, and half is getting shorter. In reality, though, only the direction the arrows are pointing changes.