bēi shuǐ chē xīn
杯 水 车 薪
用一杯水去救一车着了火的柴草:use a cup of water to put out a burning cart of faggot.it means that sth is far from enough to solve a problem，indicates there is no use，in vain.
从前，有一个樵夫砍柴回家，他经过一家茶馆，因为天气太热，于是，他把马车停到外面，就进去要了一杯茶，没多久，就听到外面大喊：“不好了，着火了”，樵夫端起茶杯就冲了出去，他把茶水泼向着火的柴草，然后又进去倒了一杯，接着泼，就这样跑了几个来回，想要灭火，结果，柴草都烧成了灰烬。
杯：cup, here it is a measure word ( a cup of)
一杯水：a cup of water.
水：water, in the legendary story related, actually it refers to a cup of tea.
车：cart，here it is a measure word means (a cart of)
薪：in old days, it refers to faggot or firewood that could be used as fuel, nowadays 薪 means salary or wage（薪水 in Chinese） .
用……去……: use…to do…
救：save
着火🔥：on fire
柴草：faggot or firewood