Hi Gerardo,

I think you missed the joke. It's a quote from "The Princess Bride". I suggest you check here: http://www.imdb.com/title/tt0093779/quotes

I apologize if you misinterpreted it but it was just one of those inane rules of thumb that was used in the film (I don't know if it was in the book itself). I could just have well mentioned "Never get involved in a land war in Asia", though, that one, as noted, is slightly more well known :).