Web Comics: Harmless or Massive Time Sinks?

I have a few web-based comics that I check on regular if not daily basis. Some are still in production, and one is all archive material.

Today's link is to xkcd. I'm not a physics/math/science nerd by any stretch of the imagination, but this comic continuously hits my funnybone and my curiosity.

Make sure to check the mouse-over texts.