Lately, I have been addicted to stereograms. Anyone has heard of it before? Well it's basically a picture with lots of random pattern in it. But if you look it at a certain way, a 3-D image will pop out. It's really amazing how our eyes work. It took me hours to be 'enlightened' and gain the ability to see the hidden 3D image though.
One example:
http://www.vision3d.com/sghidden/shark.html
if viewed correctly, the image in the link above will display a 3D shark.
Now before you accuse me of losing my mind or trying to waste your time, I am NOT. It is for real
Here's a tutorial from youtube how to see stereograms:
http://www.youtube.com/watch?v=kgrUfUI0Y9g&feature=related
I believe different people learn to view stereograms differently. The way I see it is, while looking at the image, I looked down on my nose but not letting the image escape my vision. Then after focusing on my nose, I will slowly look back at the image, and voila! The 3D image appears (most of the time).
Good luck for first-timers!
One example:
http://www.vision3d.com/sghidden/shark.html
if viewed correctly, the image in the link above will display a 3D shark.
Now before you accuse me of losing my mind or trying to waste your time, I am NOT. It is for real

Here's a tutorial from youtube how to see stereograms:
http://www.youtube.com/watch?v=kgrUfUI0Y9g&feature=related
I believe different people learn to view stereograms differently. The way I see it is, while looking at the image, I looked down on my nose but not letting the image escape my vision. Then after focusing on my nose, I will slowly look back at the image, and voila! The 3D image appears (most of the time).
Good luck for first-timers!
Unfortunately, not until a few years ago, I wasn't able to see the illusion within the pictures cause I didn't know how to look at it. Well a few years ago from now, I actually do what we're supposed to do, and I thought I was tripping out when the wanted result was seen. They can be quite entertaining once you finally know how to see them .
The first ones I ever tried would have two dots at the bottom of the page underneath the picture, and if you could get your eyes to cross enough for the dots to match up, then that was the amount you needed to cross your eyes to get the picture to work. Once you start crossing your eyes to look at these pictures, I find in my case anyway that there's a certain point where the image "latches into place," and it's actually a bit difficult to un-cross your eyes from it sometimes 

!