Toyota's 'five whys' method stops problems at their root

Developed by Toyota founder Sakichi Toyoda and implemented in the Toyota Production System, the 'Five Whys' technique asks 'why' repeatedly until the root cause of a problem is found — usually in five iterations. When a machine stopped working, the first 'why' revealed a blown fuse; the fifth 'why' revealed that metal filings from a nearby machine had contaminated the oil because a filter was never installed. Fixing the symptom (fuse) would have meant the machine broke again in days; fixing the root cause meant it never did.

Source: Ohno, Toyota Production System (1978)