We’ve done this one several times previously, though it’s next to impossible to search for. The upshot is some study found people tend to overlook some or all of the Fs in the occurrences of the word “of” and therefor undercount.
I completely missed the “OF” F’s in my initial readings of the sentence. It’s like they didn’t even exist, and I was* looking *for them. It’s interesting how your mind scans text when reading. It really is more of a sort of a mental gestalt process than a linear “read” of the text.
It appears to be a shortened version of this exercise which is intended to demonstrate the difficulty in obtaining accurate data under pressure.
Unfortunately, many websites have taken it and distilled it down to the version in the OP and frequently conclude with some variant of the ridiculous claim that the human brain cannot process the word “of.”
I teach phonics to little kids, and always point out that “of” breaks the rules, what with an “o” which sounds like a “u”, and an “f” which disguises itself with a “v” sound. So it didn’t get me.
I’ve heard this used as a demonstration of being left or right brain dominant - if you’re left brained and vocal you read it in your head and “of” sounds like “uv” and doesn’t register as an F, whereas if you’re right brained you visually look for the F so you count it.
Edit: It’s probably BS.
Strangely, I knew what the gimmick was, and quickly visually scanned the sentence and still got 3 because I ignored “of” - weird.