I like that one, too. It gave me an idea, so I found this image of a Zen Garden sandbox (creative commons, freely distributable) and Photoshopped it so it looks like somebody used the little rake to make a gear, or machine cog in the sand. I don't think it's
too obvious, but it sort of represents the intent of the forum:
Too bad it can't be animated, I could make the gear turn. OK, maybe it's not such a bad thing that it can't be animated.
EDIT: Also, I'm no Photoshop guru, so if someone else can do a better job, I'd be glad to send or link the full size image.