Mulla Nasrudin

Deductive reasoning

   ‘How old are you, Mulla?’ someone asked.
   ‘Three years older than my brother.’
   ‘How do you know that?’
   ‘Reasoning. Last year I heard my brother tell someone 
that I was two years older than him. A year has passed. 
That means that I am older by one year. I shall soon be 
old enough to be his grandfather.’