Elbow oil has the best polishing effect. China proverb. Metaphor as long as you are determined and willing to work hard, how difficult things can be successfully completed. Song Zhu Mu's "Yu Fangsheng, Lan Mei, Zhou Mo, Zhen Xi" Source: According to legend, Li Bai once studied in the mountains, but he abandoned his studies before he finished. "After the stream, you can grind the iron pestle when you are old. Ask him,' I want to be a needle.' Taibai feels its meaning and returns to school. "