Raymond Smullyan. Alice in the country wit (№ 170211)

In advance, we can say that to whom you asked whether he spoke always the truth
or a liar, the answer will always be the same: "No."
Why? - surprised someone from the hotel.
"Because," answered Lillian, - that the one who says always only the truth, never lies and will not impersonate a liar, and a liar never admits honestly that he is a liar. And in that and in other case both answer the question the same way: "No!".