Event Bubbling

When a user performs an action suh as clicking an element on a Web page, that event originates within the given element. If an event handler has been added to the element, a function may run in response to the event. However, elements are nested within other elements; in addition, all page elements are ultimately nested within the document element itself, and the document is nested with in the browser window.

In the internet explorer event model, events are initiated at the bottom of the document tree and rise to the top of the object hierarchy in a process known as event bubbling. Thus an event passes lowest object in the tree, through every object in the path to the top of the tree has an opportunity to respond to the event.