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The exasperated shout," I can only do one thing at a time!" usually comes from people attempting to do six things at once, at a time when they are only capable of doing five. One way we do five things at once is by interleaving the activities. We dice the chicken while waiting for the water to boil, turn down the heat on the rice while frying the onions, but wait a moment to take out the bread because the white sauce is just starting to thicken.

Whether we are preparing dinner, debugging a program, or getting married, a good part of our mental energies are spent linearizing. The many parallel tracks of activities must be organized into a single linear stream of actions to be performed. 1 In a sense we are very skilled at scheduling multiple activities: It is a fundamental process which is a constant part of our mental life. But at the same time, we make a lot of scheduling errors: We let the bread burn while stirring the sauce, or let the sauce get lumpy while removing the bread from the oven., The field of ergonomics attempts to design objects that take into account the realities of the human body-if a keyboard is too high, the typist's wrists will have to bend and typing will be uncomfortable. In