The way I always saw it was, most the other IFs we had the pleasure of seeing being created were more or less "thought of" before hand, so when they were officially created, their physical characteristics were already in place. So they just "popped" into existence. When Terrence was creating Red, it was a last minute decision, so he didn't have all Red's characteristics thought up of yet. So he was still becoming concrete even after he was created into "official" existence. Concerning Goo, she's soooooooo imaginative, even with the multitude of friends she's created, she's able to think up their characteristics in a moment and *pop*, there they are, not to say that some of them were premeditated, too. And I agree with Sparky, the Pizza friend was an entirely subconscience thing. I don't think he even realized it was a
friend. He just looked, and there was a slice of pizza, and without giving it a second thought, he grabbed it and ate it. And maybe he was just "slow" in realizing something was out of whack when that slice of pizza said "Howdydoo! I love you!"
