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Recently, he expressed his frustration about his teacher and classmates labeling bitcoin as a ponzi scheme or a scam each time he talks about it. I'm seeking a variety of advice that I can offer to my nephew in this situation. Although his father suggested that he should stop discussing bitcoin, I don't feel comfortable with that approach. What are your thoughts on this matter?
I thought I will be sleeping after reading your story, but I didn't.

Kidding aside, say to your nephew to stop interrupting with other people if they are saying that Bitcoin is a ponzi scheme or scam. Say to him to just focus on learning more about it than focusing on what others are saying to it. Your nephew knows that Bitcoin isn't a scam, and these people who are saying it are the ones who have little to no knowledge about it. One thing is that, the one that's saying that is his teacher. What do you expect? Your nephew will make an argument with your teacher regarding Bitcoin being a ponzi scheme? Your nephew making arguments with your classmates regarding that topic would still be good, but with a teacher? Just imagine that OP.
TBH, I'd agree with what your father said. He just stop discussing Bitcoin to other people especially if he don't know everything about it. He's too young for that TBH, but since curiosity got him to that point, I suggest to you OP to say to your nephew to just focus on learning, ignore the bad noises, move forward.