How To Correct A Child That Lies Too Much I Mean An Extreme Liar

 i tried this successfuly on my cousin when she was  little, i hope it still works now.
 give that child an exaggerated account of the fable " sheep that was always crying wolf".
       In the story ,a a certain sheep was fond off calling for help pretendind that a wolf was on him. he did it the first time ,got people running to his rescue only to tell his concerned friends that he is joking, he does did continuously until people get used to his false alarms and don't act on it. it is at this point that a real wolf attacks him. even when he cries out 4 help, no one shows up because they think he is lying again .
  MORAL ; WHEN WE ALWAYS LIE, NO NE BELIEVES US EVEN WHEN WE ARE TELLING THE TRUTH
  Describe what happened, in very vivid terms, the fate that befell the sheep @ the end of the story. my cousin was in a shock for days after that story so when i start suspecting she is lying ,i just remind her of the story and out the truth will come spilling.
 now she is 15 and tell her younger ones , or any potential liar that story .

all the best