Notice how many red balls are in this photo.
(You can click on this photo to enlarge it.)
Did you say five red balls? Are you sure?
Look again. Can you see another? Look closer. Are you positive there are only six balls?
When you scroll down, you'll discover where you went wrong. However, the longer you look at the photo and try to figure it out, the more surprised you'll be when the secret to this illusion is revealed.
Did you notice that each hand has an extra finger?
This is a real photo of the hands of Cuban man Yoandri Hernandez Garrido, who has polydactyly, which means he has extra digits on his hands (he also has an extra toe on each foot.) His fingers are so well-formed it's hard to notice that he has extras.
This optical illusion messes with our perceptions of normal. We doesn't spend much time focusing on his hands, because our brain takes the shortcut of seeing his hands as merely hands, not as six-fingered hands.
Telling you to look at the balls further takes attention away from the hands. (There are only five balls, by the way.)
This is called "inattentional blindness", where we can't see what's right in front of us because we're not paying attention to it.
It's also about not always knowing what's true about the world:
"One day when I was in primary school, a teacher asked me how much was five plus five?" Hernandez recalled. "I was very young, kind of shy, and I didn't say anything. She told me to count how many fingers I had, so I answered, "12!"- Cuban man '24' proud of his 4 extra fingers, toes, Associated Press, Yahoo News>>
"The teacher was a little upset, but it was the truth," he said.
- This man’s hands are no optical illusion, io9>>
- Inattentional blindness, Wikipedia>>