This is an old puzzle but you would be surprised how many people don't do well at it.
So, how are YOUR brains?
Read the following sentence:
Now count aloud the F’s in the rectangle.
Count them ONLY ONCE – do not go back and count them again.
There are six F’s in the sentence.
If you counted only three the less we say about it the better.
If you counted four you are above average intelligence.
If you got five you will never forgive yourself for having missed one.
If you counted all six you are brilliant and should not be wasting your time on foolishness such as this.