Look at the following animated gif:
http://nivea.psycho.univ-paris5.fr/A...kayakflick.gif
There should be two pictures that alternate. There is an interesting phonemenon that affects the majority of people, they will not be able to immediatly see the change between the two frames. When they realise what is changing it will become incredibly obvious.
Have a go, tell me if it works/how long it takes you.