What you can carry through an airport is strictly controlled by the U.S. Department of Homeland Security and its internal agency, the Transportation Security Administration (TSA). A walk through its checkpoints will uncover whether you have restricted items in your bag, which will have to be removed before boarding any aircraft.
But there’s no rule against bringing a piece of paper with you as you pass through to the airport’s gates. One woman, presumably about to board a flight of her own, carried with her a letter that she had spent some time penning by hand.
She was never meant to deliver that message to anyone in particular though. Instead, a stranger, a man she didn’t know, found her note left in the airport, and the words she had written on that page left him – and the internet at large – shocked.