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 hesitated and pulled the note back out. Messing with his own day felt too risky. He decided to ask the slot a safer question instead, and wrote: 'What should I do today?' Then he held his breath and fed it in.

The slot answered with one line: 'Teach someone else before you forget.' So Edwin walked Rosa through every detail of the magic mail. The next morning his memory of it was foggy, but Rosa remembered everything, and the slot's secret was safe for one more day.