What is a good example of a paradox?

[I_am_not_mafia]

Grelling–Nelson paradox

A word is autological if it describes itself.

A word is heterological if it does not describe itself.

Is "heterological" a heterological word? If the answer is 'no', "heterological" is autological. This leads to a contradiction, for in this case "heterological" does not describe itself: it must be a heterological word. But if the answer is 'yes', "heterological" is heterological. This again leads to a contradiction, because if the word "heterological" describes itself, it is autological.

Is "heterological" a heterological word?

   no → "heterological" is autological → "heterological" describes itself → "heterological" is heterological, contradiction
   yes → "heterological" does not describe itself → "heterological" is not heterological, contradiction