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Mine was that scene in "Witness" where the grandfather sits the little boy on his knee with the policeman's gun in front of them on the kitchen and explains why they believe guns are wrong.
It marked me so profoundly. I hadn't been brought up with explanations but it instantly changed my outlook on parenting. I didn't even have children at the time ( I think I was 18 when I saw it) but I've always tried to emulate the parenting style.