Through the magic of youtube con videos I've learned that not only is De Lancie (Q)'s favorite episode of TNG he was in was All good Things and that the scene with a little puddle of goo was the last shot for the entire series but because they had to be high up for the low angle shots there was a man below them catching the goo in a bucket as De Lancie played with it. The little smirk Q gives is because he can hear the goo hitting the guy in the face instead of the bucket.