On his first morning of retirement, Edwin Marsh swept the old Hollowbrook post office one last time. Behind a coat rack he found a brass mail slot in a wall that never had one. The little plate read TOMORROW'S DEPARTURES. As a joke, he scribbled a grocery list and fed it in. By noon, the milk he'd written down sat on his porch, in a glass bottle stamped with tomorrow's date.

Edwin got greedy. If the slot could send a grocery list to tomorrow, maybe it could send a real message. He wrote a note to himself: 'Edwin, don't sell the house. Trust me.' His hand hovered over the slot.

Edwin shoved the note in before he could change his mind. The slot clicked. A second later, a reply slid out in his own handwriting: 'Too late. You already did. Fix it.' Edwin stared at words he hadn't written yet, his heart pounding.