We talked about 3D objects being square from one side and circle from the other - for example a cylinder. But he rejected this approach (though he was able to visualize the form). He considered taking circle and square apart and putting it back together into something like a rounded square but rejected that too as neither square nor circle.
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.
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