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.

It’s not real hungernot like the painful hunger of starving people in impoverished countries. It’s more of a mild ache, or an itch that you mustn’t scratch. To be popular, a diet must somehow cope with this hunger. Weight Watchers does it with peer support. The food pyramid does it by encouraging you to eat unlimited celery. Some high-fat diets satisfy all your old cravings-and figure you’ll eventually cut back the butter you put on your bacon.

Last April, I had once again grown out of my belt. I wasn’t grossly overweight: 205 pounds in a six-foot, one-inch body. That wouldn’t be bad for a football player, but I’m 59 years old, and the excess pounds weren’t in muscle. I had gained a pound a year for several decades. I felt heavy and old. I decided to try conservation of energy. I gave up lunch and snacks.

How to cope with the hunger? I attempted to enjoy it. I thought of the movie Lawrence of Arabia, in which T.E. Lawrence says, “The trickis not minding that it hurts.” I told myself that the mild ache was only the sensation of evaporating fat. That interpretation has some basis in physics. When you lose weight, most of your fat is converted to the gases carbon dioxide and water vapor, and so you get rid of fat by breathing it out of your body.

Physics works, and I lost weight. By August, I was down to 175 pounds, a 30-pound drop. My belt went from 42 inches to 36 inches. My Zen-like approach to hunger also worked; I found myself declining offers of chocolate cake because I didn’t want to lose the sensation of evaporation. I didn’t change my level of activity, and managed to maintain my diet while taking trips to Cuba and Alaska-and during a week-long backpacking excursion in the Sierra Nevada. A key innovation: I kept up the social aspects of lunch, without eating. I watched others gobbling cheeseburgers, while I sipped diet cola. It really wasn’t that hard to do. And the mild afternoon discomfort was compensated by several positive developments. Dinner became truly wonderful. I hadn’t had pre-dinner hunger for decades. A sharp appetite turns a meal into a feast. No more cheese “appetizers” for me.

Moreover-and this may sound silly coming from a physicist-I was surprised that I began to feel lighter. I no longer walk down streets-I float. Distant stores seem closer. And my knees have responded to the lighter load. Their aching, which I had mistakenly attributed to aging, went away.

Food is instant gratification. And fast-food chains and gourmet restaurants serve tasty food at remarkably low cost. It is a situation unprecedented in history and unanticipated by our genes. No wonder we are overweight.

Anybody can lose weight. Energy is conserved. Just stop scratching that itch. Of course, you’ll have to sacrifice instant gratification. Is it worth it? You decide. Food is delicious and cheap. You might reasonably choose to take advantage of this unique historical circumstance, and decide to be fat.

It’s been seven months since I started my diet, and two months since I left it. I’ve begun eating a light lunch, and having an occasional small snack. I’m still at 175. But I never want to lose the delicious edge of hunger before dinner, or the floating sensation when I walk. Moving takes less energy now, so I have more energy. I no longer feel like a spherical physicist. And for losing weight, dieting sure beats cleaning animal stalls.