Catch the Dog
written by Martijn Overmars

Goal
The goal of this extremely simple game is to catch the dog with the mouse. Each time you manage to click on the dog, you get a point. 

Playing
When the dog appears, quickly move there with the mouse and click on it. If you are fast enough you get a point.

Design
This is probably the simplest game you can thing of. It has just one object for which two events play a role: the alarm event when we move the dog to a random place, and the mouse event when the player manages to click on the dog.