Here's an old joke. The dairy industry hires a physicist to improve milk production. After several weeks, he's ready to lecture about his progress. He draws a circle on the blackboard and says, "Consider a spherical cow."

I've told this joke many times, but nobody ever laughs-except other physicists. For the rest of you, I should explain that it is self-deprecating humor. It makes fun of our penchant for oversimplification.

This month I want to talk about diet and exercise for weight loss, and I'm going to oversimplify on purpose. Consider a spherical physicist.

Most dieters are so concerned about second-order effects, such as daily fluctuations in weight and changes in metabolism, that they lose track of the first law of thermodynamics: conservation of energy.

Want to lose a pound of fat? You can work it off by hiking to the top of a 2,500-story building. Or by running 60 miles. Or by spending 7 hours cleaning animal stalls. (It is amazing what scientists have actually measured. This last example is tabulated in the book Exercise Physiology by G. Brooks and T. Fahey.)

Exercise is a very difficult way to lose weight. Here's a rule of thumb: exercise very hard for one hour (swimming, running, or racquetball) and you'll lose about one ounce of fat. Light exercise for an hour (gardening, baseball, or golf) will lose you a third of an ounce.  That number is small because fat is a very energy-dense substance: it packs about 4,000 food calories per pound, the same as gasoline, and 15 times as much as in TNT.

If you run for an hour, you'll lose that ounce of fat and also a pound or two of water. By the next day, when you've replenished the water, you might think, "the weight came right back!" But you'd be wrong-you really did lose an ounce. It is hard to notice, unless you keep running every day for a month or more, and don't reward yourself after each run with a cookie.

There is a much easier way to lose weight, as we can learn from the first law of thermodynamics. Eat less.

A reasonable daily diet for an adult is 2,000 food calories. That's 8.36 megajoules per day, or about 100 joules per second-in other words, 100 watts.  Most of that ends up as heat, so you warm a room as much as a bright light bulb. Cut your consumption by 600 calories per day and you'll lose a pound of fat every week. Most diet experts consider that a reasonable goal. Don't drop below 1,000 calories per day, or you might get lethargic. But at 1,400 calories per day, you can easily maintain an active life.

Of course, there is a catch. You'll be hungry.