It’s not obvious because, as stated, it’s wrong: https://arxiv.org/abs/1612.05833 https://arxiv.org/abs/1612.05833 You can prove that the disk and the square ar

It’s not obvious because, as stated, it’s wrong: https://arxiv.org/abs/1612.05833 https://arxiv.org/abs/1612.05833 You can prove that the disk and the square are not scissors-congruent, based on a convexity invariant, but if you allow more exotic dissections you can construct an equidecomposition.