Aorist rods were devices used in a now happily abandoned form of energy production. When the hunt for new sources of energy had at one point got particularly frantic, one bright young chap suddenly spotted that one place which had never used up all its available energy was -- the past. And with the sudden rush of blood to the head that such insights tend to induce, he invented a way of mining it that very same night, and within a year huge tracts of the past were being drained of all their energy and simply wasting away. Those who claimed that the past should be left unspoilt were accused of indulging in an extremely expensive form of sentimentality. The past provided a very cheap, plentiful and clean source of energy, there could always be a few Natural Past Reserves set up if anyone wanted to pay for their upkeep, and as for the claim that draining the past impoverished the present, well, maybe it did, slightly, but the effects were immeasurable and you really had to keep a sense of proportion.

It was only when it was realised that the present really was being impoverished, and that the reason for it was that those selfish plundering wastrel bastards up in the future were doing exactly the same thing, that everyone realised that every single aorist rod, and the terrible secret of how they were made would have to be utterly and forever destroyed. They claimed it was for the sake of their grandparents and grandchildren, but it was of course for the sake of their grandparent's grandchildren, and their grandchildren's grandparents.

-- Young Zaphod Plays it Safe by Douglas Adams 1986