I love these! Growing up, there was one in my house and it took me years and years to figure it out.
In these I saw some lions, a camel, some weird snowmen, and ants (maybe?). For anyone who doesn't know, take of your glasses if you wear them, get the image out of focus, and then slowly bring your eyes back into focus. If you do it right, you'll see a 3d image emerge from the noise.
Additionally What you are going for is, if you hold up lined paper so the lines are going vertically, to let your eyes out of focus so that you would start seeing double, ((as if looking at something through and passed the page) and then focus your eyes where the doubled lines of the page overlap. You can then play with bringing into focus lines that are two lines apart, three lines apart, 4 and so on. The action of seeing past the page and focusing on the doubled image made by the overlapping is what you are going for.
