I learned a lot about binocular vision in elementary school. Staring at the cinderblock walls day after day, it was bound to happen eventually that my eyes would lose focus and then find it again in the wrong place, making the whole wall shimmer as my brain struggled to make sense of the conflicting details.

Stereograms are easy to draw, and even easier to create using digital photography. Just take a pair of photos, one a few inches to one side of the other, overlay them in the computer, and cut away the parts that don't match up. Especially easy if you can put the camera on a nice flat surface while taking the picture.

Here's one I took of the beautiful icicles cascading from the roof while snowed in on Presidents' Day 2003. I'm not sure what causes the bands in the icicles - maybe the thermostat kicking in and helping to melt more snow from the roof every so often?

This is the "wall-eyed" version, the same type used in stereogram viewers. If this one looks bizarre when you focus it, try the "cross-eyed" view below.

This is the "cross-eyed" version. I've never learned how to view these properly.