Five Whys is a technique used to explore the cause-and-effect relationships underlying a particular problem. Each "Why?" question should be the basis for the next question. "Five" was derived arbitrarily as the number of times to ask questions to arrive at the root cause, but it is possible to identify a root cause with fewer or greater than five questions.
The last answer should point to a process failure or defect. This is one of the most important aspects in the "5 Why" approach - the real root cause should point toward a process that is not working well or does not exist.
A key phrase to keep in mind in any Five Why exercise is "people do not fail, processes do."