This post contains spoilers....

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The little balls of Goo that are crashing around the title screen after the World of Goo corporation is destroyed (or maybe only after the Epilogue?) are actually normal balls, it seems, and can be manipulated.  It's a little hard to see because both the balls and the selection cursor (the four black dots spinning around) are smaller here than in the game.

The balls can be selected, moved around, and even thrown in an orbit around the World.  They're all free balls though, I've not seen any pre-existing Goo structures to build on.  I've yet to find anything to hook them onto, like one can in the window within the MOM level.

Anyway, it's a nifty little extra!

Yep, it's a neat little secret. I knew about it already, but hopefully this topic will let some more people learn the trick. It's pretty cool.

That's not even secret and their their from the beginning of the game so it isn't a spoiler.  ::)

Started a new game today and noticed that, lo, Macattack is right.  Ah well, I thought it was cool anyway.