What would happen if you take a magnet to a pickups sound through an amp? Would it kill the pickup or have any affect at all?
A simplified picture of how an electric pickup works is that: the magnet in the pickup induces the string (steel/nickel alloy) to be a magnet itself. When you pluck the, string, the magnetized string moves relative to the coils in the pickup. Moving a magnet near a circuit causes a voltage across the circuit (or equivalently a current for a closed circuit). This voltage is then amplified by the amplifier. The complications involve the fact that the magnitization of the string is higher when the the string is closer to the pickup, and less when further away, so the idea of the string just "being a magnet that moves around" doesn't capture the whole picture. The key point is that moving the string around changes the magnetic field that the coils experience, and thus produce a voltage.
Moving any kind of magnet near the coils will induce a voltage across them.
For a hand held magnet, it would probably be hard to move the magnet fast enough to produce a sound (you'd have to shake it back and forth 10's of times per second), but a strong enough magnet moving fast enough might produce enough voltage to overload an amplifier if it is connected.
Holding the magnet near the pickup while moving string will either enhance (if the magnet is aligned such that it is attracted to the pickup) or degrade the gain in the pickup since the external magnet's field will add to the pickup's field either "in phase" or "out of phase". Horse-shoe style pickups have magnetic elements both above and below the strings to affect the "shape" of the magnetic field near the strings.
One of the methods a person may use to magnetically charge a pickup pole when winding your own pickups is to stroke the pole with a strong magnet and the pole becomes magnetized itself by this action. With that thought in mind, I would advise caution about playing with a valued pickup and an external magnet, because you can also reverse the polarity and or weaken the poles magnetic field with out meaning to. I often encourage experimentation as a learning tool but I'm also interested in not causing harm to an instrument. Please take care when you fool with your pickups They are quite fragile.