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  • Andrew Deck thinks this is interesting:

Typical items in the home that an adventuresome person might attempt to repair include a door lock, toaster, and washing machine. The device is apt to have tens of parts. What has to be remembered to be able to put the parts together again in a proper order? Not as much as might appear from an initial analysis. In the extreme case, if there are ten parts, there are 10! (ten factorial) different ways in which to reassemble them—a little over 3.5 million alternatives.


Cover of The Design of Everyday Things: Revised and Expanded Edition


10! is actually low. It assumes that you're just lining the components up, and that orientation doesn't matter. Even if you are just adding components one-by-one, adding orientation gives roughly infinite possibilities. Then assuming that there are configurations such that the only correct way to snap parts together is to combine three or more at once, that also multiplies the set of possible configurations pretty markedly.