Visual Illusions - Attention

How many F's did you count? Three?

WRONG ANSWER !

There are six F's in the text. Take a look:

FINISHED FILES ARE THE RE-
SULT OF YEARS OF SCIENTIF-
IC COMBINED WITH THE
EXPERIENCE OF YEARS.

The explanation is:
The human brain pays little attention to words that are not very important for understanding the sentence, like the word 'OF'. Moreover, the fact that the sentence is written in capital letters makes it harder to distinguish the F's. If you counted the correct number of F's you are very special. (It is also possible that you stared at your screen for a very long time.)