The Evening Wall

Every evening, a quiet apartment wall began tapping exactly three times. The sound faded when she leaned close, then returned a minute later. She checked the pipes, moved a bookcase, and called the building manager. He listened carefully, but the tapping returned after he left. On Friday, she stayed late and noticed it began at sunset. She switched off the heating, and the wall fell silent. The next day, she watched the thermostat as dusk settled outside. Its timer clicked, and warm water rushed through pipes behind the wall. The tapping came from heating pipes expanding whenever the evening timer switched on.