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Mercury coating was a fairly common science experiment and does leave the coin somewhat magnetic.
Mercury coating is NOT a fairly common science experiment and it does NOT leave the coin magnetic.
Most likely the coin is NICKEL plated. Nickel is highly magnetic but typically there isn't enough nickel there to make it more than slightly magnetic. It will attract to a magnet, a magnet will pull it around and may even be able to pick it up. But a true steel or nickel coin will JUMP to the magnet as soon as it gets close enough. (Depending on the strength of the magnet the coin will jump anywhere from a quarter to half an inch.)