Someone discovered that animal grease or fat rubbed into the inner side helped the leather to become soft.
有人发现，动物油脂或脂肪擦到皮革内有助于皮革变得柔软。
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The material choice is simple: the shoe is a laminate with Carbon fibre for the core which gives the shoe its spring and strength, leather on the foot side and rubber on the walking wearing side.