It started last week with a simple question in the Canadian city of Edmonton.

Air traffic controllers there wondered how they could help their counterparts in Anchorage, with whom they frequently communicated over the airwaves - and who were closing in on three weeks of a U.S. government shutdown, working without pay.

The Edmonton air traffic controllers opted for a universal symbol of goodwill: They sent pizzas.

It was a "small gesture of kindness," the president of the Canadian Air Traffic Control Association would tweet later, but a "big gesture of solidarity."

Soon, word spread and other Canadian airports and air traffic control centers followed suit.

On Friday, a flier appeared in the hallways of the air traffic control center in Ronkonkoma, New York, with an all-caps header ("PIZZA!!") and a note explaining that about three-dozen pies would be arriving soon, a "show of solidarity and support" from air traffic controllers in Moncton and Gander, two small airports in Atlantic Canada.

By Sunday, Canadians had sent more than 300 pizzas to at least 40 U.S. facilities, according to the Canadian Air Traffic Control Association.

"It was a free-for-all, people going crazy sending each other pizzas," David Lombardo, a former trainee at the Ronkonkoma air traffic control facility, told The Washington Post.

This weekend, Canadian air traffic controllers ordered hundreds of pizzas for their American counterparts working without pay due to the ongoing US government shutdown https://t.co/vANgp3DPeM — CNN (@CNN) January 14, 2019

Lombardo, who now runs the satirical Facebook group ATC Memes (“it’s like ‘The Onion’ for aviation”) and who first posted an image of the pizza flier to Reddit, said he wasn’t surprised because air traffic controllers in Canada and the United States are very close, constantly “handing off” planes to one another.

"It's kind of a unique situation because these are co-workers you may never see in your life but nevertheless you're working with them hand in hand," he said. "The aviation industry is a huge family, it really is. You feel like you're part of something much bigger than yourself, but at the same time, it's really tiny. The international boundary doesn't mean much when it comes to airspace."

Peter Duffey, president of the Canadian Air Traffic Control Association, said the effort - which a few Twitter users dubbed #pizzadiplomacy - was entirely organic.

He told CNN that, while the effort started out between facilities that work closely together, some pairings have been unlikely: Air traffic controllers in Fort McMurray, Alberta, for example, sent pizzas to El Paso, because they are both oil towns.

During this shutdown, more than 24,000 Federal Aviation Administration employees have been working without pay, while more than 17,000 have been furloughed, including those who train and support air traffic controllers, The Post’s Michael Laris reported.

On Friday, the first day many federal employees in the United States missed a paycheck because of the shutdown, the National Air Traffic Controllers Association filed a lawsuit against Trump and other officials for forcing them to work without pay.

Missed paychecks are an unnecessary added stress in a job where “we have to be 100 percent correct, 100 percent of the time with zero room for error,” Duffey told HuffPost Canada. Sending pizzas was the least the Canadians could do for their American colleagues, he added.