Stereoscopy is a technique where the illusion of depth is artificially created: slightly different images are presented to the left eye and right eye. 3d glasses merge these two separate images, so that your brain can process them as a single (seemingly) 3d image.
Without 3d glasses on, things start to look a bit…dizzying. Here are some striking examples of stereoscopic photography.
1. Sophie, the hot dog. By jimf0390.
1. This dress might be sold out, but we can still dream about it.
Here’s a tutorial on how to make your won stereoscopic images.