If you work at it hard enough, you can grind an iron bar into a needle.; cf idiom 磨杵成針|磨杵成针, to grind an iron bar down to a fine needle (idiom); fig. to persevere in a difficult task; to study diligently
trad. 只要功夫深，鐵杵磨成針
Also written 只要功夫深，鐵杵磨成針【zhǐ yào gōng fu shēn , tiě chǔ mó chéng zhēn】