Sensational - the try 'em at home guide to illusions

The Size-Weight illusion

This illusion, first described over 100 years ago, shows that if you lift two objects of equal weight, you will tend to perceive the smaller object as heavier. You can test this out at home. Take two empty plastic bottles and using scales fill each with water or sand so they both weigh the same. Ask a friend to lift them up and ask which they think is heavier. If they say the smaller bottle then the 'size-weight' illusion is at work. You can even tell them the objects are the same weight. The illusion will still persist.

But things get even stranger! Researchers have found that when people alternately lift objects of the same weight but different size they apply the same fingertip force to both objects even though they still experience the size-weight illusion. One part of their brain is being fooled but another part of their brain isn't, and the two parts aren't communicating with each other.