Illusions

The monthly magazine Geo of this month has as lead theme "Illusions". The main point were optical illusions. Our brain plays games with us, what we think we see is not what is real.
In this picture (modified to make it my own) most people see waves, although there aren't any, not in the paper and not in the painting. It's just the pattern that suggests our brain to interpret the signals from our eyes as wavy.