The Pulfrich effect is a really cool optical illusion! It can be easily demonstrated with a pair of sunglasses with one lens missing. When a pendulum swings back and forth, it swings in a straight line. But your eyes can be fooled into thinking it's moving in a circle! This effect expands beyond the pendulum, as we show.

Want some 3D glasses so you can try this at home? Click the "3D Glasses!" link on the right side of our blog page, or visit "http://www.cmmap.org/everydayscience/" and let us know where to mail your glasses! (After it's mailed out, we don't save your contact information. We promise!)