You must complete a level with the cooperation of your past and future selves by using your TARDIS-like time machine. Each level takes just a couple of minutes, and I've only been stumped once (the video walkthroughs are helpful). If you loved Portal, you'll like this enough for two lunch breaks.
Length: :30-5:00 per level; 35 levels
When I gave up: Level 28
Pretty goodness: Pretty pretty good