Call it what you like: A gut feeling. A sharp eye. A sixth sense developed from logging thousands of air miles. But something strikes many travelers as odd about how all airplanes have ashtrays in their lavatories. After all, smoking has been banned from domestic U.S. flights since 1990 (and now on international flights too), and flight attendants routinely read the riot act to passengers about how they should never ever smoke on the plane.

Yet, there it is, in every bathroom: a convenient ashtray, customarily located on the inside of the door.

At the very least, ashtrays on no-smoking airplanes seem like a mixed-message extravaganza, somewhat akin to the concepts of a "healthy tan" and "a vitamin donut." The placement of a no-smoking sticker next to each ashtray adds to the cognitive dissonance.

Over the years, fliers have floated a few theories about the ashtrays. (Hat tip to blogger and frequent traveler Matt Simmons for re-surfacing this topic recently.) Perhaps most airplanes are old and pre-date the change in smoking laws, so it's not affordable for airlines to rework the tooling and uninstall them. To which we respond, seriously? It's too expensive to remove ashtrays?

Another theory making the rounds is that Boeing and Airbus manufacture airplanes in a one-size-fits-all process, and it's not cost effective to build some planes without the ashtrays. Yet considering that there's no major airline in the world that allows smoking, this theory doesn't seem plausible. Plus, every large airline buys aircraft interiors customized to their needs, and it would be weird if ashtrays were some great exception that couldn't be excluded.

To clear the air on this matter, we turn to the pros.

It turns out that airlines are required by law to have an ashtray on the lavatory doors of every plane, thanks to a Federal Aviation Administration (FAA) regulation. In a directive updated as recently as 2000, an ashtray is part of the"minimum equipment," meaning that if something happens to any one ashtray, an airline must replace it within three to ten days, depending on the circumstances. (In case you were wondering, the European Union has similar regulations.)

So what's the theory behind the law? An FAA spokesperson (who requested not to be named) clarified for us by e-mail: "Even though smoking is banned, many people continue to smoke on airplanes. This provides a place to put a cigarette, other than the waste bin, or somewhere less desirable where the ashes might start a fire."

Ross Cheit, an associate professor of political science and public policy at Brown University and author of Setting Safety Standards: Regulation in the Public and Private Sectors, weighed in too: "Even with strict rules against smoking and with sensitive smoke detectors, we know that some people will, on occasion, break the rule. If there are safe disposal methods, breaking the rule will not lead to a serious fire risk, which could well happen if, say, someone disposed of a cigarette in, say, the towel dispenser."

Yet aren't the ashtrays an incitement to breaking the law?

Cheit responds: "I don't see how this can be said to encourage smoking since the smoke detectors are both very apparent and extremely sensitive. You really cannot "'get away" ' with smoking in the lavatory without getting caught. But again, when the airline stewardess is knocking on the door because the smoke detector was triggered or tampered with, one does not want the violator to do anything with the cigarette that could cause a serious problem."

Ah, well. Those other theories were blowing smoke. But don't feel bad if your suspicions about the reason behind the ashtray didn't pan out. Take comfort that you know you're well traveled when you perceive the slight anomalies, such as when your economy-class seat belt has an air bag (hint: it's in that bulky box on top) or when your business- class seat has a water bottle holder in a slightly different position from the norm. So if you also noticed that lavatories have ashtrays, congratulations. You've got great instincts, deep powers of observation, and a savvy bred from logging a lot of travel time.

Photo: Najlah Feanny/Corbis