You are in a concrete room. There is a steel pipe 25 centimetres in length cemented into the centre of the concrete floor. The pipe protudes about 15 centimetres. A Ping-Pong is dropped down the pipe. There is a fraction of a centimetre clearance around the ping-pong ball and the pipe. Your task is to get the ping-pong out of the steel pipe undamaged. The only items that are available are a wooden ruler, a ball of string, a pocket mirror, a paper clip and a small magnet. Since nothing else is allowed into the room, how could you get the plastic ping-pong ball out of the steel pipe?
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