The idea is the same as Mad Libs, but not trademarked. It is a game best played by a group of people, but it can be fun for just two people. One player will be referred to here as "the moderator."
There is a very short story with some blanks in it. Each of the blanks has a description of what sort of word should go in the blank. Sometimes the description will just be what part of speech the word should be ("noun, plural" "adjective," for example). Other times the description will be more specific (such as "breakfast food" or "temperature adjective").
The moderator reads the descriptions to the other players and writes the words they suggest to fill in the blanks. Once all the blanks are filled in, the moderator may read the story or assign another player to read it. The story might not necessarily make sense from a narrative point of view, but it should make sense grammatically, for the most part.
This game would be played with pen and paper, it still often is today. The Mad Libs publishing company published books with these stories, on pages that could be torn off, if I recall correctly. Presumably people would keep the better stories and throw away the rest.
With computers, it is possible to do several Wacky Fill-Ins with zero paper waste. You can also reuse a particular story and fill it in differently. And if you do like a story enough to want it on paper, you can print it out.
For this project, I also want to enable users to write their own Wacky Fill-In stories. See index.html for details.