Some years ago a test pilot was flying a new model of jet. At one point in the flight he heard an unusual, non-mechanical sound. He looked down at where the sound was coming from to see a rat gnawing through one of the cables. He realised that this was not good. He could not deal directly with the intruder himself as he was engaged in flying the plane. If he aimed to return to his airstrip he was fairly certain that the rat would finish its task first, probably with catastrophic results. His only option was to deprive the rat of what it most needed; oxygen.