watermill(mill powered by water) noun [UK: ˈwɔː.tə.mɪl] [US: ˈwɔːr.tə.mɪl]
杯水車薪 noun
drop in the bucket(an effort or action having very little influence, especially as compared to a huge problem) noun [UK: drɒp ɪn ðə ˈbʌkɪt] [US: ˈdrɑːp ɪn ðə ˈbʌkət]