This one took me a little time to get - but pretty neat when you see it:
The gif shows a series of dots in a circle that can be made to either change color or vanish completely depending on where you look.
Confused? Just choose one of the moving spots. To make the dots change color, follow the light grey spot around the circle as it moves. After around 30 seconds, you're likely to see the rest of the dots turn a teal/green shade. To make all of the dots completely disappear, just stare at the cross in the center of the circle for 30 seconds.
See the dot? And no it's not an animated gif.