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In Goblet of Fire Chapter 14 Professor Moody showed Harry's Defense Against the Dark Arts class the Unforgivable Curses. He used spiders to demonstrate the curses.

Is there any particular reason why he needed to use a different spider for each of the three curses? Why couldn't he just have done the Cruciatus Curse on the same spider that he had done the Imperius Curse on, and then done the Killing Curse on that same spider as well?

Even if he did not know in advance which order he would demonstrate the curses in, he would have only needed two spiders in case he did the Killing Curse first.

So is there any particular reason why separate spiders were necessary? Would the effects of the curses not been the same, or not as apparent, if the spider had just been subjected to a different Unforgivable Curse?

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    Probably not, but hey, don't we go complaining about one of the only guys in the wizarding world who shows some respect for unit testing principles... :p – Jenayah Jul 27 '18 at 17:54
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    I've never thought of this before! – Simpleton Jul 27 '18 at 17:57
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    It seems fairly logical, actually -- he's demoing three different spells, and wants to show what each does, with no there being clearly no contamination from the previous spells; i.e., a Crucio'ed spider could be killed more easily, one might suppose. Using three just keeps the results clean in an obvious way. And with lots of spiders available, no real cost. – K-H-W Jul 27 '18 at 17:58
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    @K-H-W Except to the spiders, that is. ;-) – Janus Bahs Jacquet Jul 27 '18 at 18:05
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Using three spiders makes clear each is the effect of one spell.

While he could have used the same spider for all the Unforgivable Curses, using three different spiders is a good way to make it clear to the students that the effects caused are solely the result of the spell just cast on it, not a cumulative effect of multiple spells. We know that casting multiple spells at the same object can sometimes change the effect that would happen if only one was cast. For example, when Harry, Ron, and Hermione all tried to disarm Snape (though they were cast all at the same time, not one after the other), instead of just disarming him, they knocked him out and caused a blast - neither of which are typical effects of Expelliarmus.

Expelliarmus!’ he yelled – except that his wasn’t the only voice that shouted. There was a blast that made the door rattle on its hinges; Snape was lifted off his feet and slammed into the wall, then slid down it to the floor, a trickle of blood oozing from under his hair. He had been knocked out.

Harry looked around. Both Ron and Hermione had tried to disarm Snape at exactly the same moment.”
- Harry Potter and the Prisoner of Azkaban, Chapter 19 (The Servant of Lord Voldemort)

We do know the Cruciatus Curse leaves its victims with residual pain.

“And these words burst from Harry’s mouth; they echoed through the graveyard, and the dream state was lifted as suddenly as though cold water had been thrown over him – back rushed the aches that the Cruciatus Curse had left all over his body – back rushed the realisation of where he was, and what he was facing …”
- Harry Potter and the Goblet of Fire, Chapter 34 (Priori Incantatem)

Using three different spiders makes it clear that the effects the students are witnessing are solely the effect of the spell that was just cast. Even if ‘Moody’ knew it wouldn’t make a difference if he used the same spider, the students had no experience with the effects of Unforgivable Curses, so it made for a clearer demonstration. In addition, it wouldn’t have been particularly difficult or resource-consuming to get two more spiders, so there was really no reason why not to.

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