看美剧学英语习语：（start）from scratch

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（start）from scratch: Entirely without the aid of something that is already prepared or in existence. Refers to making something, usually food, from the raw or base ingredients or components, rather than those that have been preassembled or already partially completed.
Fig. [making something] by starting from the beginning with the basic ingredients. (*Typically: bake something ; do something; make something; Start (something) .)

中文释义 :（start）from scratch中scratch 作名词解的时候有“起跑线”的意思。start from scratch 直译为“从起跑线开始”，引申出来也就是“从头开始”、“白手起家”的意思了。例句： he baked the cake from scratch.  (他从零开始烤蛋糕。)she built her business up from scratch. (她做生意是白手起家的。)