Here's a fun optical illusion.
When you click on "Start" you will go to a program which generates a moving pattern.
When you launch the program, stare at the small dot in the middle of the pattern for 30 seconds.
Then look at the back of your hand.
P.S. The pattern is generated by a program called "optical.exe" which you will have to launch to view.
Don't worry, it is virus free.
When you are through, press ESC to close and exit the program