Sally saw it first. There was a dark streak down the side of the water tower where water had run and dried and run again over a long time. The tower was at the end of the street, past the lumberyard, too far to walk to easily. They could see it from the corner. "It leaked," Sally said. Dale looked at it. The streak was wide at the top and narrower as it came down, fading before it reached the bottom of the tank. The tank itself was old, dark green where it wasn't streaked, sitting on four legs above the lumberyard roof. "It didn't leak," Dale said. "Towers don't leak. They're pressure vessels." Sally looked at the streak. "That's condensation migration," Dale said. "When the temperature differential between the water inside and the air outside reaches a certain point, moisture moves through the wall." "Through the metal," Sally said. "Along the metal," Dale said. "It finds the low path. That's what moisture does."

Sally looked at the streak. It was clearly coming from a seam near the top. Dale looked at it from a slightly different angle, though they were too far away for the angle to change much. "The seam concentrates it," Dale said. "That's not a weakness. That's where the tower moves the differential out. By design." "They made it do that," Sally said. "They designed for it," Dale said. "There's a difference." Sally looked at the seam. A pigeon landed on the rim of the tank and walked along it and flew off. "How full is it," Sally said. Dale looked at the tower. "You can tell by the sound," Dale said. "A full tower has a lower resonance. The water dampens it." He listened toward the tower for a moment. It was too far away to hear anything. "It sounds full," he said.

Sally looked at the streak again. It was dry now in the afternoon light but the staining went all the way back, layer on layer, each run leaving its own faint line inside the wider one. "It's been doing that for a long time," Sally said. "Same conditions, same result," Dale said. "That's how you know the system is stable." Sally counted the lines inside the streak without moving her lips. Dale looked at the tower for a long moment. Then he looked around for something closer to examine, some part of the system he could actually reach. There was nothing. Just the street and the lumberyard fence and the tower at the end of it. He looked back at the tower. "Good tower," he said. But he said it quieter than usual.

He turned and started back up the street. Sally stayed at the corner. The tower stood at the end of the street the way it had always stood, old and streaked and full or not full, above the lumberyard in the afternoon light. She looked at it a while longer. Then she followed him.