I was working on a somewhat complicated image mask of a motocross rider the other day at work. Whenever I work on complicated things like this, I try to save every few minutes to make sure that if Photoshop crashes, I don't lose much of my work. When I was done, I had a short progression of images that showed my progress as I made the mask, and I thought it'd make a neat animation. Check it out:

In case you're not into computer graphics, an image mask is a special bit of extra data attached to a layer of graphics, represented in grayscale, where black represents total transparency, white represents total opacity, and the values in between make up the range from transparent to opaque. Basically, it's a way to tell Photoshop (or other image editors) where you should be able to see through parts of an image. Masking is obviously a very important part of working with computer graphics.