In Smullyan’s island there are knights who always tell the truth and knaves who always lie. Knights and knaves cannot be distinguished by their looks. In each problem your goal is to understand, whether possible, if the person talking is a knight or a knave.

Can an inhabitant of the island say << I am a knave>>?
No he cannot, because a knight can only say that he is a knight; but if someone asks the knave to which category he belongs to, he will reply lying that he is a knight.