When in the flow state, people lose track of time and the outside environment. They are at one with the task they are performing. ... The flow state occurs when the challenge of the activity just slightly exceeds our skill level, so full attention is continually required.

Constraints are powerful clues, limiting the set of possible actions. The thoughtful use of constraints in design lets people readily determine the proper course of action, even in a novel situation. ... The value of physical constraints is that they rely upon properties of the physical world for their operation; no special training is necessary.

Knowledge how—what psychologists call procedural knowledge—is the knowledge that enables a person to be a skilled musician, to return a serve in tennis, or to move the tongue properly when saying the phrase “frightening witches.” Procedural knowledge is difficult or impossible to write down and difficult to teach. It is best taught by demonstration and best learned through practice.

Design redundancy and layers of defense: that’s Swiss cheese. The metaphor illustrates the futility of trying to find the one underlying cause of an accident (usually some person) and punishing the culprit. Instead, we need to think about systems, about all the interacting factors that lead to human error and then to accidents, and devise ways to make the systems, as a whole, more reliable.

A conceptual model is an explanation, usually highly simplified, of how something works. It doesn’t have to be complete or even accurate as long as it is useful. ... Simplified models are valuable only as long as the assumptions that support them hold true.