Got this via email today, I haven't seen it before, it's really weird!! (sorry if it's already been posted - haven't been around recently) 
How many F's are there in the following text?
FINISHED FILES ARE THE RE-
SULT OF YEARS OF SCIENTIF-
IC STUDY COMBINED WITH THE
EXPERIENCE OF YEARS ...
(see below)
Managed it ?
Scroll down only after you have counted them, okay?
Do you think there are three?
Wrong, there are 6 !!
Read it again.
The reasoning behind it is further down.
The brain cannot process "OF".
Incredible or what ? Go back and look again!!
Anyone who counts all 6 "F's" on the first go is a genius.
Three is normal, four is quite rare.