xkcd, Devotion To Duty 167
xkcd really hit the nail on the head today.
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What determines the strip's shape is its differing areas of "energy density," they say.
"Energy density" means the stored, elastic energy that is contained in the strip as a result of the folding. Places where the strip is most bent have the highest energy density; conversely, places that are flat and unstressed by a fold have the least energy density.
Invest in physics -- own a piece of Dirac!