Sometimes you can't tell a person not to panic because if they see their hard earned money turned into ashes they will likely to panic. I recommend them to cut some losses if it is still small.
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If we know we can't handle the risk in price increase/decrease in crypto then we might not try to do trading. If we think that the project seems no longer future to increase in price then sell. But if in every short drop of price even in a potential coin, we panic, then trading might not be for us.