[Thanks to Kirk]
1.- Click on this link.
2.- Then "click me to get trippy",
3.- Look at the center of the screen for 30 seconds (no cheating), and then
4.- Look at your hand holding the mouse, without moving it away from the mouse.
"Why do you doubt your senses?"
"Because the slightest thing might affect them."
(A Christmas Carol.)
Paul Neave has made other nice stuff.