Stereograms!

[Kerog]

I saw snot's. I can see these sometimes, but the one you made is hard to see.

[Ad Bot]

[Spockticus]

I'm fairly convinced that these things work purely on the power of suggestion. 

Yeah, I can never see stuff like this. I just hurt the hell out of my eyes from snot's.

[VerticalHorizon]

It takes practice. You have to cross your eyes slightly so that the image doubles and the patterns match up, some stereograms use images (Like dots, squares, or other small images) to help because they're easier to line up.

[snot2]

the muscles in my right eye make it hard to move it left and right so i can cross my eyes easily

actually i cant move it right at all