Permanent magnets with diminished field strength can be recharged in at least three ways: by striking, stroking or stacking. Each of these methods entails lining up the weakened magnet with a stronger external field and using it to realign its particles.
Striking a magnet involves aligning the magnet with the Earth's magnetic north pole. Once aligned, the magnet can be forcibly struck with a hammer to shake its molecules into parallel lines. Stroking involves repeatedly running a strong magnet over the weakened magnet to pull its particles into alignment. Stacking strong magnets together with the weakened one, then clamping them together for a period of time, also strengthens the weak magnet's field.