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  • Double Liar Paradox (Jourdain’s paradox)

    The following is written on opposite sides of a card:

    Front side:
    THE SENTENCE ON THE OTHER SIDE OF THIS CARD IS FALSE.

    Back side:
    THE SENTENCE ON THE OTHER SIDE OF THIS CARD IS TRUE.

  • Liar Paradox (Epimenides Paradox)

    1. A Cretan sails to Greece and says to some Greek men who are standing upon the shore: “All Cretans are liars.” Did he speak the truth, or did he lie?
    2. A week later, the Cretan sailed to Greece again and said: “All Cretans are liars and all I say is the truth.” Although the Greeks on the shore weren’t aware of what he had said the first time, they were truly puzzled.

    If someone says “I always lie”, are they telling the truth? Or are they lying?

  • Fly and trains

    Two trains, 200 km apart, are moving toward each other at the speed of 50 km/hour each. A fly takes off from one train flying straight toward the other at the speed of 75 km/hour. Having reached the other train, the fly bounces off it and flies back to the first train. The fly repeats the trip until the trains collide and the bug is squashed.
    What distance has the fly traveled until its death?

    Answer

    If the fly is flying for 2 hours still at the same speed of 75 km/h then it flies a distance of 150 km.

  • Brick weight

    One brick is one kilogram and half a brick heavy.
    What is the weight of one brick?

    Answer

    1 brick = 1 kg + 1/2 brick
    And so 1 brick is 2 kg heavy.

  • Honestants and Swindlecants I

    There are two kinds of people on a mysterious island. There are so-called Honestants who speak always the truth, and the others are Swindlecants who always lie.
    Three fellows (A, B and C) are having a quarrel at the market. A gringo goes by and asks the A fellow: “Are you an Honestant or a Swindlecant?” The answer is incomprehensible so the gringo gives another quite logical question to B: “What did A say?” B answers: “A said that he is a Swindlecant.” And to that says the fellow C: “Do not believe B, he is lying!”
    Who is B and C?

    Answer

    It is impossible that any inhabitant of such an island says: “I am a liar.” An honestant would thus be lying and a swindlecant would be speaking truth. So B must have been lying and therefore he is a swindlecant. And that means that C was right saying B is lying – so C is an honestant. However, it is not clear what is A.

  • Headscratching riddle

    What is greater than God,
    more evil than the devil,
    the poor have it,
    the rich need it,
    and if you eat it, you’ll die?

    Answer

    Nothing.