This is an enhanced edition of HBR article 99509, originally published in September 1999. HBR OnPoint articles include the full-text HBR article plus a summary of key ideas and company examples to help you quickly absorb and apply the concepts.The Toyota Production System is a paradox. On the one hand, every activity, connection, and production flow in a Toyota factory is rigidly scripted. Yet at the same time, Toyota's operations are enormously flexible and responsive to customer demand. How can that be? After an extensive four-year study of the system in more than 40 plants, the authors came to understand that at Toyota it's the very rigidity of the operations that makes the flexibility possible. That's because the company's operations can be seen as a continuous series of controlled experiments. Whenever Toyota defines a specification, it is establishing a hypothesis that is then tested through action. This approach--the scientific method--is not imposed on workers, it's ingrained in them.