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NancyLebovitz comments on Open thread, Jan. 18 - Jan. 24, 2016 - Less Wrong
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My guess is that your son doesn't have a solid grasp of the idea of a cross section. Actually, I don't quite feel good about a cylinder having a square cross section. It's as though it's wrong to neglect the idea that a cylinder is round.
Consider a square in front of you with its edges horizontal and vertical. (Say, drawn on your monitor.) Then consider the line running from the top of the square to the bottom of the square that passes through the center of the square. What happens when you rotate the square around that line?
That was also my first impression. But we talked about it a bit longer. I think it clicked when he mentioned how he looked (in imagination) at the form such that the top becomes a straight line (like looking a paper from the side) and the same with the bottom.