I read that copper gives them electric shocks ---why?
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The slime that snails secrete when they move supposedly react chemically with the copper to give the snail an electrical shock. Snail slime will also react with nitrate ions that can result in the creation of substances toxic to the snail and that will kill them.
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copper is a good conductor. On cold days, it might shock the snails just as on cold days, our car door might shock us.