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I like self-referential jokes like that one, and the sign that reads: It is forbidden to throw stones at this sign.

Here is another one I saw in an illustration of a British-born rabbi’s office in Berlin. It may well be a Hassidic joke from the beginning of time, but it was new to me.

Apart from silly jokes like these, self-reference is not a particularly amusing literary contrivance: Wikipedia has a rather boring article about it. But it does include some good examples, such as "This sentence contains threee erors". Are there only two? In that case the sentence is an error in itself, in which case there are three errors, in which case it is a true statement, in which case.... I believe there is a special name for this kind of circular confusion, but I cannot think what it is.