If you were to break a rear wheel of your car, at first it would seem almost impossible to get home. However, by taking a pole and placing it under the car, the same as you would on a wagon, fastening to the rear axle or to the running board, then tying your rear wheel to the pole that is broken so that it cannot turn, you will ascertain with pleasure that you can drive home with the other wheel, since it is bound to turn. This is due to the differential which allows one wheel to stand still while the other one runs twice as fast.

 Found on the Horseless Carriage Web Page
http://www.hcca.org