I can't stop staring at this optical illusion that I came across today. Each ball is traveling back and forth in a straight line, but as more balls are added, they begin to look like they are bouncing around in a circular motion. The longer I stare at it, the harder it is to believe that the balls are staying on their lines.
Here is an interactive version of the illusion: "Interactive Dot Illusion (Individual Linear Motion Yields Circular Motion)". Users can adjust the number of dots, the dot speed, and the dot color. The faster It goes, the weirder the illusion looks. It's pretty mesmerizing, and fun for both kids and grown-ups to play around with.