This is a test used by the CIA and FBI to screen applicants for field work. There are three differences between the two pictures. You have 90 seconds to notice the differences. Less than .1% of people get all three. The answers are recorded on audio at the end of the test, so make sure your sound is on (I didn't have my sound on the first time and sat there forever waiting for the answers . . lol)
http://members.home.nl/saen/Special/Zoeken.swf