http://en.wikipedia.org/wiki/5_Whys

The technique was originally developed by Sakichi Toyoda and was later used within Toyota Motor Corporation during the evolution of their manufacturing methodologies.

The five whys is a question asking method used to explore the cause/effect relationships underlying a particular problem. Ultimately, the goal of applying the 5 Whys method is to determine a root cause of a defect or problem. The following example demonstrates the basic process:
My car 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 has never been replaced. (fourth why)
Why? – I have not been maintaining my car according to the recommended service schedule. (fifth why, root cause)

Cool! I ask that all the time, ha. No wonders Toyota Motor Corporation is set to be no. 1 automaker in the world soon.