An optical illusion (also called a visual illusion) is characterized by visually perceived images that differ from objective reality.
Here is one amazing painting by Oleg Shuplyak.
How many people can you see in the picture below?
I see five (5) human faces
I believe it's 5
Depending on how you look at this image, you may see a portrait of a woman, or a silhouette of a man playing a saxophone.
There are many flutes hidden in this picture below.
How many Flutes you can see ?
How many faces do you see in the picture below?
