5 Whys

The 5 Whys is an iterative question-asking technique used to explore the cause-and-effect relationships underlying a particular problem.

The primary goal of the technique is to determine the root cause of a defect or problem.

The "5" in the name derives from an empirical observation on the number of iterations typically required to resolve the problem.

Example:
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)
Why? - Replacement parts are not available because of the extreme age of the vehicle. (sixth why, optional footnote)

Start maintaining the vehicle according to the recommended service schedule. (possible 5th Why solution)
Adapt a similar car part to the car. (possible 6th Why solution).