Two pounds of apples cost more than one. Mailing a big, fat envelope is more expensive than mailing a letter. Smokers pay more for life insurance than do non-smokers.

So why shouldn't you pay an airline according to how much you weigh and how much space you take up?

A report the other day from the National Center for Environmental Health tells us that our collective national belt-loosening is costing the airlines big money -- an extra $275 million in fuel costs in one year to account for the 10-pound average increase in Americans' weight over the past decade.

The airlines are desperately trying to cut fuel costs by cutting the weight they carry -- replacing metal utensils with plastic ones, scrapping heavy magazines. But the solution lies directly beneath the floor of the passenger cabin: Charge passengers by the pound, just as freight in the cargo hold is priced.

That would not only help the airlines, but more important, would create a social and financial disincentive for becoming or staying obese.

As things stand, the one-third of Americans who are not overweight subsidize the two-thirds who are. It's in everyone's interest to shift the balance back toward healthier, slimmer lives.

The last time I took up this quest, a couple of years ago, I had the sad duty to report that L.L. Bean had changed its pricing policy and would charge the same for its clothing no matter the size. It was a big break for the big-boned, and a strike against fairness for everyone else. Traditionally, clothing that's made of more material has, logically enough, cost more than the same item in a smaller size.

But compared with L.L. Bean greedily chasing after the loyalties of its broadening customers, the transportation industry faces a tougher predicament. After all, an airplane seat is only so wide.

Advocates for the obese -- yes, they have a Washington lobby, too -- reject solutions that put the responsibility for obesity squarely on the bellies of the big. The American Obesity Association argues that our commercial and entertainment culture bears a good part of the blame for our becoming a nation of wide loads.

With almost any other malady, we distinguish between people's behavior and their affliction. You might behave in ways that make it more likely that you'll get cancer, but when you do get cancer, it's generally perceived as a really bad break that deserves oceans of sympathy. But if you're large, the public reaction is that it's your own darn fault and maybe you should lay off the Mars bars. "We suspend the compassion that we normally feel," as Morgan Downey, director of the obesity group, put it. Instead, we laugh in the general direction of fat people, making a "moral judgment of laziness, lack of self-control, weakness," Downey said.

Fair enough. But accepting the idea that obesity is a community problem actually strengthens the case for creating such disincentives as paying by the pound for air travel.

Yet when Southwest Airlines tried to draw the line against corpulent passengers flowing over onto someone else's seat, the obesity lobby rolled into action. Southwest's policy is simple and fair: If you take up more than one seat, you need to buy the extra seat. The airline refunds the extra charge if there are empty seats on a given flight.

The Obesity Association reacted by asking Southwest to install wider seats for the extra-large. No way, said Southwest, which went ahead with its policy, noting that only six seats per airplane account for their profit margin. Replacing only three rows of seats per plane with extra-wide seats would suck up all their profits, Southwest's president argued.

All that extra weight we're carrying is a burden not only on the airlines' bottom line, but on our very survival. After a commuter plane crashed last year in Charlotte, federal investigators said it might have been overloaded. So the FAA ordered airlines to revise the formula they use to estimate the weights of passengers. The feds tacked an extra 10 pounds on to the assumed average weight of an American adult.

The answer: Public weigh-ins before every flight. Heck, it'll add some entertainment while we wait at the security checkpoints. Shame, a nearly-lost tool of social suasion in this society, is good: Line 'em up, weigh 'em in and watch the pounds come off.

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