Alice wanders in the Forest of Forgetfulness where she cannot remember the day of the week. She meets the lion and the unicorn. The lion lies on Monday, Tuesday, and Wednesday and tells the truth the other days, while the unicorn lies only on Thursday, Friday, and Saturday.

"Yesterday was a day when I lied," says the lion.

"Yesterday was a day when I lied," says the unicorn.

Question: What day is it today?
The sought day cannot be Monday, Tuesday, or Wednesday. Let's consider the case of Monday (the other two days are eliminated in a similar manner): if it is Monday, then the unicorn is telling the truth and the lion is lying. So the unicorn is right in saying that the previous day, it was lying. However, the day before Monday is Sunday, and on Sunday, the unicorn tells the truth, so the sought day cannot be Monday (this contradicts the fact that the unicorn is right!).

We can eliminate Friday, Saturday, and Sunday in the same way (by analyzing what the lion says this time) so we are left with Thursday. We can also verify the consistency of the lion's and unicorn's statements on Thursday to ensure our reasoning is correct!