In the movie Labyrinth, Sarah has to reach the castle, located in the center of the labyrinth, in order to get back her kidnapped baby brother. On her journey to the castle Sarah faces, amongst others, the following challenge:
There are two doors and two guards, one who always lies and one who always speaks the truth. One door leads to the castle in the center of the labyrinth and the other door leads to certain death. The guards know which road leads to the castle. The riddle is to find out which door leads to the castle by asking one of the guards a single yes-no question.
Sarah solves the riddle by asking the following question:
Would he---Sarah points at the guard of the right door---tell me that this door---Sarah points at the left door---leads to the castle?
Upon hearing the guard's answer, Sarah argues that she knows which door to take. For sure, Sarah's reasoning is correct, but in virtue of which principles? In Playing with Truth, various "riddles about truth" (invented by Raymond Smullyan) receive a formal treatment. Besides guiding Sarah through the labyrinth, we also show how she can solve the Hardest Logic Puzzle Ever.