The label of almost any food package today will tell you the calorie content. But how are calories determined? Is it very technical? –Steven Weinstein, Brooklyn, New York

Oh, sure, you’ve got your beakers and test tubes and stuff like that. But when you get right down to it, they measure calories in food the old-fashioned way–they burn it. At least they did years ago, in a wonderful device known as a bomb calorimeter, presumably so called because its centerpiece, a thick-walled metal can with wires leading out of it, would send ’em running for the exits if somebody found it in an airport locker. You put the food inside, torched it, and measured the total heat output. You thought maybe “burning off the calories” was just a figure of speech? Uh-uh. Your body burns food just as the calorimeter did, though admittedly in a less dramatic manner.

Researchers don’t use calorimeters much today because years of experiment have reduced calorie calculations to a simple formula: protein and carbohydrate each have four calories per gram and fat has nine, regardless (more or less) of what food they’re found in. Aha, you’re thinking–I know my multiplication tables, I’m qualified to be a food scientist. Not so fast. The trick is figuring out the amount of protein, fat, and carbohydrate in each food. That’s where the beakers and test tubes come in. You want to hear about the oxidation of sugars by an alkaline solution of trivalent bismuth in the presence of potassium sodium tartrate? I didn’t think so. But the underlying premise of calorie computation is simplicity itself.

A MATTER OF LIFE AND DEATH

Your answer to the question about the doors of death and life [December 13] needs attention. The question was, “You’re in a room with only two doors. One door leads to death, the other to life. Each door has a guard. One guard always tells the truth, the other always lies. How do you get out by asking one guard one question?” Your answer was to ask one guard, “If I were to ask you if your door was the death door, would you say yes?” where “yes” would mark the death door. No! The correct question would be, “If I were to ask the other guard if his door led to death, what would he say?” where “yes” marks the life door. I’d have those tonsils removed if I were you. –Katie MacInnis, Pflugerville, Texas

Katie, you may be the bright light of Pflugerville, but this is the big leagues. My question works fine. So does yours, except for one detail–“yes” marks the death door, not the life door. Let’s chart the possible answers to your question:

“If I were to ask the other guard if his door led to death, what would he say?”

Truthful guard

Life door: Other guard has death door but lies so truthfully says no

Death door: Other gurad has life door but lies so truthfully says yes

Lying guard

Life door: Other guard has death door so falsely says no

Death door: Other guard has life door so falsely says yes

No matter which guard you ask, he says yes if he’s in front of the death door. Now let’s chart my question:

“If I were to ask you if your door was the death door, would you say yes?”

Truthful guard

Life door: Would say no if asked, so says no now

Death door: Would say yes if asked, so says yes now

Lying guard

Life door: Would falsely say yes if asked, so falsely says no now

Death door: Would falsely say no if asked, so falsely says yes now

Get it? Either guard says yes if he’s in front of the death door, same as with your question. If you don’t mind, I’ll leave my tonsils where they are.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.