杯水车薪

bēishuǐ-chēxīn

[a cup of water can't put out the fire on a carload of wood; try to put out a burning cartload of faggots with a cup of water-an utterly inadequate measure] 用一杯水去扑灭一车燃烧的柴草。 比喻力量太小, 无济于事

犹以一杯水, 救一车薪之火也。 --《孟子·告子上》