There are a few cases where you want to know if the user is idle. Namely: You want to preload more assets You want to grab their attention to pull them back You want close their banking session after 5 minutes of inactivity. (Jerk!) You want the site to sneak off the screen and see if they notice ;-) Nick Zakas wrote a script for YUI3 to handle these cases. His writeup has a great description of t