why are farts so funny?

this is a question that i have been pondering for many a year.  can anyone help me with this one?  farts are in fact very funny but i don't know exactly what makes them funny.  my wife claims that farts are only funny to people under the age of 5 but i know that that is not true.  look, i am twenty something and i still find farts to be very, very funny.  

i think it might have something to do with the fact that a noise is coming from the butthole and the butthole is sometimes considered to be a private area and for some reason the discussion of private areas is funny.  

i don't know.  all i know is that nothing is more likely to bring a smile to the face of an individual than that of a nice, juicy fart.  They're just funny, that's all there is to it .

But why?