Wonders: Laws of Calorie Counting; August 2000; Scientific American Magazine; by Philip Morrison, Phylis Morrison; 2 Page(s)

Gazing into our quiet street, we see a scrap of paper ruffle in the breeze or a little water creep along the gutter. Those motions have clear causes: wind and gravity. But the local raccoon that hunts by night and the car that rolls past are distinct. The forces that impel both are mustered internally: timed muscular contractions lift and plant paws, and a stream of explosions turns the wheels. Each selfmover draws on a diet of energy, the one scraps of food, the other gasoline.

The quantity of energy we take daily from food has entered common parlance in the U.S. Every edible offering on the shelf declares by law its nutritional energy in calories-units of heat, a form of energy release easily measured. (Bottled water declares itself out of the energy game: 0 calories per serving.) One kilocalorie is heat enough to raise the temperature of one kilogram of water by one degree Celsius. (Many labels carry the term "calorie" but all the same intend by it the kilocalorie.) The energy we expend is neither provided as nor mainly used as heat.