ring

Try this...Think about a math problem while looking at it.

Then remember something like a hot bath.

This is more fun than 3-d pictures, same principle, I believe.

The trick is to focus your eyes at a point 12 to 18 inches away from your face, hold your finger there as a reference point if need be, pull your hand away without letting your eyes try to re-focus as they normally would.

Takes a bit of practice.