Read the following sentence:
FINISHED FILES ARE THE RESULT OF YEARS
OF SCIENTIFIC STUDY COMBINED WITH THE
EXPERIENCE OF YEARS.
Now count how many F's does that sentence contain. Count them ONLY ONCE. Then see below:
The sentence contains 6 F's. However, most of the people only find 3 of them. If you spotted 4, you're above average intelligence. To see 5 is quite rare. If you caught 6, you might be a genius. There is no catch or a trick. Most people forget the F's in of''s because the human brain tends to see them as Vs instead of F's.