A root-cause analysis technique: when a problem occurs, ask "why?" five times (or as many times as needed) to trace the causal chain from symptom to root cause. taiichi-ohno's example: Why did the machine stop? A fuse blew. Why? Overloaded. Why? Bearing not lubricated. Why? Lubrication pump not working. Why? Pump shaft worn. Fix the shaft, not the fuse. The technique prevents the common error of treating symptoms instead of causes. Ohno insisted on going to the gemba (actual workplace) and observing the actual process before asking why — the five whys must be grounded in observation, not speculation. The technique was adapted by Eric Ries for the Lean Startup methodology, where it is applied to organizational failures and process problems. Simple in concept but demanding in practice — it requires intellectual honesty and willingness to follow the causal chain wherever it leads.