No "permanent magnet" is completely permanent. Heat, sharp impacts, stray magnetic fields, and age all conspire to rob a magnet of its force. A magnet gets its field when microscopic magnetic areas, called domains, all line up in the same direction. When the domains cooperate, the magnet's field is the sum of all the microscopic fields in it. If the domains fall into disorder, the individual fields cancel out, leaving the magnet weak.