(Thanks to Qaqaq for writing this description)
Say Goodnight, Gracie is almost always an impromptu game, played in odd moments or to wind down an evening. It often happens in the middle of a normal conversation. There is no scoring or real end objective.
The game usually begins when one person happens to have mentioned some word or phrase. Let's say someone, in the course of an ordinary conversation, happened to say the sentence “I can't remember what her name was … I'm thinking Mandy”. For many people, this would be an immediate spur to start up SGG. They would reply “Oh, no, that's not what you're thinking. You're thinking of an animal from Florida that people want to save” (thinking “manatee”, a word relatively close in sound to “Mandy”). This will inspire someone else to say something like: “No, no, don't be absurd. What you're actually thinking of is the end of the famous exclamation, “Oh, the…” (thinking “humanity”, a word relatively close in sound to “manatee”). And on and on, in no particular order. The only rule is that the words in the chain can never actually be said.
The fun is in getting puns that are as close as possible, and clever whenever possible. This game often attracts people to jump in; it's easy to figure out what's going on even if you stumble into the middle of a game and have never seen it played before.
The game usually ends when people get sick of it, or when it's time to do something else, or when by consensus a perfect ending word/phrase has been reached. Or some combination of the above.