几何体：骨架，决定了形状（是方的、圆的，还是扭曲的）。
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The school run was over and laundry was in the tumble dryer. Mum and step-mum of four Liz McConnell was about to sit down to work at her Dover home last September. But that's when the sound of a fire alarm cut through her morning.
// 3. 从后向前放置（保证稳定性）