You can use the EventTrigger trigger to invoke an action based on an event such as a mouse click, a page loading, or another interaction. EventTrigger is the default trigger for actions that you define in Microsoft Expression Blend.
To add an EventTrigger
In the Assets panel, click Behaviors, and then drag the behavior that you want onto the object that you want to apply the behavior to. The Trigger category appears in the Properties panel.
In the Trigger category, in the SourceObject box, select the element that will activate the event. By default, the SourceObject is the element that contains an action. You can use the Artboard Element Picker to select the element directly on the artboard.
In the EventName drop-down list, select the event that you want to trigger the action.
The events available in the EventName drop-down list will depend on the SourceObject that you have selected in the SourceObject box.
To see a sample of this behavior, visit the Expression Community Gallery .