Play the “animal” game (“20 questions” with no limit).
The user thinks of an animal and the program uses a tree whose nodes are yes-no questions and whose leaves are animals to try to guess which animal the user is thinking of. If it fails, the user is invited to extend the program’s tree by telling it a question to distinguish his animal from the program’s best guess.
Possibly inspired by/descended from the 1978 book, BASIC Computer Games.
There is an online version of the game, probably only vaguely related to the original BASIC or this Lisp version.