Squirrels.  I love squirrels but they can be very naughty in an attic.Image

Someone was chewing on the electrical cables.  This is a major safety concern because the exposed electrical wires could cause a fire.  It is also a good reminder of why upgrading to arc-fault circuit breakers is a good idea.  If the exposed wires allow a paralell arc from the hot wire to the neutral (black to white) wires, a regular circuit breaker my not shut off the circuit.