Microsoft Expression Blend 4 SDK includes new built-in behaviors, which are reusable pieces of packaged code that can be attached to any DependencyObject,
and then fine-tuned by changing their properties. Behaviors allow you to add interactivity to your applications without having to write any code.
The Behavior API consists of three core classes: Trigger, Action and Behavior.
No packages depend on System.Windows.Interactivity.WPF.