There is a door you pass through every day. Perhaps it is the entrance to your office building, the glass door at your favorite shop, or the heavy entryway to a hotel lobby. You push it open, walk through, and let it close behind you. The door swings smoothly, pauses for a moment at the center, then gently returns to its closed position. You do not think about it. You never do. But beneath that door, hidden in the floor, a mechanism is working. It has been working all day, every day, for years. It has held the weight of the door, controlled its swing, ensured it closes softly and securely. It is called a floor spring, and it is one of the most overlooked yet hardest-working components in any building.