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and remembering. Here mental models play a major role. Mental models simplify learning, in part because the details of the required behavior can be derived when needed. They can be invaluable in dealing with unexpected situations. Note that the use of mental models to remember (in this case, derive) behavior is not ideal for tasks that must be done rapidly and smoothly. The derivation takes time and requires mental resources, neither of which may be in great supply during critical incidents. Mental models let people derive appropriate behavior for situations that are not remembered (or never before encountered). People probably make up mental models for most of the things they do. This is why designers should provide users with appropriate models: when they are not supplied, people are likely to make up inappropriate ones.15

The sewing machine provides a good example of the power of a mental model. A sewing machine is a mysterious beast, managing to loop an upper thread through a lower thread, even though each thread is always connected to its spool or bobbin, respectively. The mental model has to explain how the upper thread goes through the material being sewn, dips under the surface plate, and then loops around the lower thread.

The proper model, it turns out is something like this. Picture the lower bobbin held gently in the machine by a kind of cup with sloping sides. The cup keeps the bobbin stable, allowing it to rotate so its thread can be unwound. Yet the cup is loose enough so that the upper thread can go inside the cup and around the bobbin—and therefore around the bottom thread. When the upper needle goes through the material and under the plate, a rotating hook grabs its thread and guides it between the inner walls of the cup and the outer walls of the bobbin case. This helps explain why the machine won’t work properly if the bobbin is bent, even if the bobbin still appears to fit and the bottom thread unrolls properly. It explains why dirt on the bobbin or in the cup will mess things up, and why certain kinds of upper thread might cause more trouble than others. (A thick upper thread, especially one that was rough or sticky, might not go smoothly around the bobbin.)

To be honest, I don’t know if anything I just said about the failures of bobbins is true. I derived each example from my mental model of a sewing machine. I can’t sew. But when Naomi Miyake did her research for her doctoral thesis in my laboratory, she studied people’s understanding of sewing and of the machines. The result was twofold: a fine piece of research for her and a mental model for me. So now I can derive what would happen, even if it has never happened to me.

The power of mental models is that they let you figure out what would happen in novel situations. Or, if you are actually doing the task and there is a problem, they let you figure out what is happening. If the model is wrong, you will be wrong too. Am I right about the sewing machine? Decide for yourself: go look at one.

After word got out that I was collecting instances of design peculiarities, a friend reported the following about the sunroof of his new car, an Audi. Supposedly, if the ignition is not on, the sunroof cannot be operated. However, a mechanic explained that you could close the sunroof even without the ignition key if you turned on the headlights and then (1) pulled back on the turn-signal stalk (which normally switches the headlights to high beam), and (2) pushed the close control for the sunroof.

My friend said that it was thoughtful of Audi to provide this override of the ignition key in case the sunroof was open when it started raining. You could close it even if you didn’t have your key. But we both wondered why the sequence was so peculiar.

Ever skeptical, I asked to see the manual for the car. The manual was explicit: “You cannot work the sunroof if the ignition is off.” A similar statement appeared in the discussion of the electrically powered windows. My friend’s mental model was functional: it explained why you would want such