STANDING in line at security at San Francisco International Airport not long ago, family in tow, I dutifully pulled the laptop out of my bag and placed it in a separate bin for its solo trip through the X-ray machine. I also had an iPad in my backpack, so I caught the eye of a security agent. “Excuse me, does the iPad come out too?” I asked.

“Not here,” she said. “Other airports might be different.”

This was not the moment for a follow-up question, but I was curious: What’s the distinction between the devices? Similar shapes, many similar functions, the tablet is thinner but not by much. Is the iPad a lower security risk? What about the punier laptop-like gadgets, the netbooks and ultrabooks? What about my smartphone?

Safely back at my desk, where a follow-up question wouldn’t risk triggering delays that could spread throughout the nation’s air traffic system, I began investigating what seems like an existential question for the digital age: When is a laptop a laptop?

There must be a reason the laptop is singled out as the bad boy of electronics at the airport. Or has the world of gadgets moved so quickly since 2001 when the laptop rule went into effect — and back when the tablet and smartphone were still in the incubator — that federal regulators have not kept up?