Read quickly through the following text and count the number of F's:

"Finished files are the result of years of scientific study combined with the experience of years"

PLEASE READ AFTER THE EXERCISE:

How many F's did you find: 3? Unfortunately that is not correct, it were 6 F's in the text. Try to read it again now.

Almost everyone guesses three. Why? It seems that the brain cannot correctly process teh word "Of". The letter F usually makes the "F" sound, like in "Find". However, in the word "of" it sounds more like a "v". Somehow your brain overlooks the word "of" as it scans for the sound of "F".