(This was the best existing thread to post this in.)
Lately I've been working on getting the OCDs in World of Goo. Some of them are pretty easy, others are mindboggling.
Like "The Upper Shaf" I think it's called. The one where you build down and let a bomb explode, then let your construction hang from the (green goo) wire in the shaft.
You need 45 balls for the OCD. I only got 25-30.

Until I understood you could recycle the green ones. But still not enough. Then by accident I discovered that if a constructed normal goo ball is surrounded by only green balls, and you remove the green ones, you also get the normal ones back. Now I had the ingredients to get more than 45 balls.
Very satisfying.
Does anyone else still play it?