It’s a quintessential inflight riddle — a perennial that comes on mid-red-eye and is forgotten by the time the wheels touch down. If smoking is banned on planes, what’s with all the ashtrays?

Think about it: there are illuminated signs above the seats; there is the pre-take off safety demonstration that reminds you that tampering with the smoke detectors is a criminal offense; there is the sign on the bathroom door; then, right below that sign, there’s the ashtray.

U.S. airlines started banning smoking back in the late 1980s. By the end of 1990 it was prohibited on all domestic flights under six hours in duration, according to CNN. And since 2000 the smoking ban has been pretty much ubiquitous internationally. So does that mean the planes we are flying on are all super old?

Thankfully not.

“You’re not allowed to smoke, but some people still do it,” a cabin attendant on a Cathay Pacific flight bound for London says, adding that she catches somebody about every six months. “So if you do smoke there has to be a safe place to stub it out.”

That safe place is emphatically not the lavatory’s waste bin, which is liable to be filled with flammable tissues.

There are good reasons for airlines to be cautious. In 1973, 123 passengers were killed on a plane traveling from Rio de Janeiro to Paris when the pilot made an emergency landing after the cabin filled with smoke. The suspected cause? A cigarette.

Now, ashtrays in bathrooms are listed by as a legal requirement for “minimum equipment” by the Federal Aviation Administration (FAA). So seriously is the regulation taken that in 2009 a Mexico-bound British airways flight was grounded after it was discovered not to be packing any ashtrays.

So there you have it, ashtrays on planes are not relics. Neither are they an invitation to smoke; they are basically there because the FAA does not trust you not to.

Correction: The original version of this story misstated the year a plane flying from Rio de Janeiro to Paris crashed. The plane crashed in 1973, not 1972.

Write to Joseph Hincks at joseph.hincks@time.com.