Behaviors allow you to add interactivity without writing code. Open this project in Expression Blend 3, launch the Asset Library, find the Behaviors tab, and drag/drop the behavior you want to use.

Here is an introductory tutorial explaining the motivation behind why behaviors are really useful.

Note:
This project has been updated to support the final version of Silverlight 3 and Expression Blend 3, and many behaviors you had seen before are no longer included in this project. The reason is that many of those behaviors are provided for you as part of the Blend installation itself :)