This week's illusion was discovered by Dartmouth College neuroscientist Peter Tse, author of "The Neural Basis of Free Will: Criterial Causation", and presented as a Top 10 finalist at the recent Best Illusion of the Year Contest.

The Knobby Sphere Illusion tricks your sense of touch. To experience it, you will need a regular pencil (for instance, with a hexagonal cross-section, and a small hard sphere (such a marble or ball bearing). Squeeze the pencil lengthwise very hard between your thumb and first finger for a full minute, until you can see deep indentations in your skin. Now feel the sphere by rolling it around against the parts of your fingers where the indentations are. The sphere no longer feels round, but bumpy. Your brain assumes that the touch receptors in your skin lie on a flat sheet, and misattributes the skin deformations to the sphere.