A Silver Screw

I moved along the edge of the shelf. My joints produced measured clicks. In a dim recessed corner, a small metallic cylinder rested horizontally. I halted. I lowered my chassis to inspect it. It was a threaded silver screw, approximately two centimeters in length. The helical ridges along its shaft caught the room light. I extended my right claw. I pushed the tip forward. It rolled precisely half a turn across the grain of the wood, then settled into a shallow groove. The thread pattern is uniform. I left it secured there.