底部的材料通常是由拉伸的聚乙烯材料制成，可以起到进一步的缓冲作用。这种材料和在汽车仪表盘和保险杠表面的吸能泡沫材料属于同一种材质。

Further cushioning is provided by the base material, which is usually made of expanded polyethylene-the same foamy stuff used in cars to provide shock-absorbent surfaces for dashboards and bumpers.

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