So why do you need to take a laptop out of your bag when you send it through the x-ray machine, but not a cell phone, iPad, or netbook? I have no clue, and neither does the TSA.

A spokesman said the agency has its reasons for still requiring that traditional laptops go through X-ray machines in a separate bin. But he declined to share them, saying the agency didn’t want to betray any secrets.

As I did more reporting, the logic behind the rule grew as elusive as a free power outlet in the boarding area. Is size the issue? If so, security experts counter, today’s laptops are far thinner than they used to be.

Could it be because laptops, unlike tablet computers, have an easily removable battery compartment and hard drive that could be used to hide homemade bombs? But some netbooks and ultrabooks have similar compartments, and they don’t require separate screening. Strike two.

Perhaps, I thought, it’s because the circuitry of a laptop can be replaced with a device to send an electromagnetic signal to jam an airplane’s controls at takeoff or landing. But, as I soon learned, the same circuitry could be embedded just as easily in phones, watches or game players, all of which stay in the bag.

I was starting to feel like a Monty Python character, riding a pretend horse, clomping my coconut halves together to simulate the sound of horse hooves. A comical quest for a mythical grail.

The T.S.A. wouldn’t comment, obviously, on whether laptops are better carrying cases for bombs. But the agency’s “blogger team” was on the case, having published several posts that acknowledge the potential confusion created by the popularity of so many new gadgets like digital readers and tablets.

“I’ve read many a post from people wondering if these items should be treated like a laptop and removed from their carry-on bags,” reads the first T.S.A. post on the subject, from April 2010, in an explanation signed “Blogger Bob, T.S.A. Blog Team.” Bob then writes: “Great question!”

The post explains that electronic items may stay in the bag if they are smaller than the “standard-sized laptop.” Laptops and larger electronics should come out so that screeners can get a better look at them and see more easily into the rest of the bag. Blogger Bob writes: “It’s that simple.”

Right, like quantum mechanics. About six months later, Bob chimed in again, writing a post in response to questions about the MacBook Air, a new line of slimmer Apple laptops. He reiterated the previous rules but added an extra rule related to screen size, measured in inches.

“With those rules in mind, the 11” model of the MacBook Air is fine to leave in your bag, and the 13” model must be removed prior to X-ray screening.”

So wait. Screen size is the guiding principle? At last, fellow travelers, a lead. I went back to present my smoking gun to security.

H/T MM