The "illusion" is that this is just a picture of coffee beans; but it is not.
Can you find a man's face among the beans?
Some say that if you find the man in 3 seconds or less, the right half of your brain may be more well developed than most. (I found him immediately!)
PS: This is not a trick. A man's face is really hidden among the beans.
*(From the National Institute of Environmental Health Sciences Page)