5 Whys

A tool or method typically used in a team meeting that involves asking the question "Why?" about 5 times for each cause in order to drill down to the potential root cause.  It doesn't have to be 5 questions if the team believes they drilled down to to a reasonable depth.
Related StatStuff Video: 
   -Section 5-Measure Phase, Lesson #24 - Identify Root Causes – 5 Whys