This little demo shows you a trick which
can be performed using tracking rectanges. You are welcome to use this example to create similar changing alerts or other fun things for instance in your info panel. Please give some credit where due.
The source code may also be useful in case you want to use tracking rectangles in your applications.

ChangingAlert by Dmitri Linde - author of other neat apps including TeXEdit, TeXWorks, ArtificialIntelligence, and Sniff.
Copyright 1993
contact dmitri@physics.stanford.edu