Your brain doesn’t always see what’s there – it sees what’s useful!

Although you see the central square on the top of the cube as brown, and the central square on the side of the cube as orange - amazingly they are both physically identical.

A light-meter would show that the light coming into your eyes from both squares is the same.

If you’re still not convinced, print out the cube and use paper with two small holes in as a mask.

So how can two physically identical tiles look so different?

Our visual systems have evolved to see what is useful, and this includes cleverly adjusting for shadows.

Suppose you or your distant ancestors were searching for ripe red fruit in the shadow of a tree. The actual light reflected from the red fruit would be different from in sunlight. However, to eat and survive, you need to still see the fruit as red and therefore ripe.

The cube shows off this effect. The top surface looks brightly illuminated compared to the side. Your brain therefore makes assumptions about the original colours of the squares (based on a your previous experience) and adjusts what you see.

With thanks to R. Beau Lotto