Sometimes asking ''Why?'' once isn't enough. 5 times usually is.

   The Toyota company figured out that ''5 Why's'' is usually
   enough to get to the *real* reason behind something. It's not
   that people are lying - but it forces them to think deeper. You
   can do more of them - or less - but five times in a row usually
   seems to be enough. Note: Remind me not to work for Toyota. This
   would get annoying :P

   ''The vehicle will not start. (the problem)

   Why? - The battery is dead. (first why)

   Why? - The alternator is not functioning. (second why)

   Why? - The alternator belt has broken. (third why)

   Why? - The alternator belt was well beyond its useful service
   life and not replaced. (fourth why)

   Why? - The vehicle was not maintained according to the
   recommended service schedule. (fifth why, a root cause)''

   From Wikipedia