In What Is the Name of This Book? (1986), Raymond Smullyan describes two curious denizens of the Forest of Forgetfulness. The Lion lies on Mondays, Tuesdays, and Wednesdays, and the Unicorn lies on Thursdays, Fridays, and Saturdays. Each tells the truth on the days it doesn’t lie.

One day Alice encounters the two of them resting under a tree. They tell her:

Lion: Yesterday was one of my lying days.

Unicorn: Yesterday was one of my lying days too.

“From these two statements, Alice (who was a very bright girl) was able to deduce the day of the week. What day was it?”