If you've ever wondered what makes a car or machine run, check out this site:
The animations of combustion, steam and stirling engines are really fun and interesting to watch, especially for anyone interested in mechanical engineering.
Matt Keveney, the site's creator, created schematic drawings of each engine using DesignCAD, made copies at set intervals to simulate the animation, exported each copy as a separate image, and combined the images into an animated .gif file. This page explains how he did it.
You can get similar models using Sketchy Physics, more on that later!