Suppose you are visiting an island with knights who always tell the truth, knaves who always lie, and jokers who can do either.
Along comes an islander and says: “I am a knave.”
What is the islander?
Answer
The islander is a joker.
A knight cannot lie and say they are a knave, a knave cannot tell the truth and say they are a knave and a joker is free to lie and say they are a knave. The islander has to be a joker.