by 后面 加某种工具 或通过某种方法去完成某一件事情；有 “用、通过”的意思
by +n./+V-ing/反身代词
例如：I go to work by car .（用汽车这种工具上班）+n.
I do my homework by myself. (自己一个人完成作业) +反身代词
The girl makes money by saling flowers. （通过卖花这种方法途径挣钱）+v-ing