望梅止渴 (wang mei zhi ke)
"quenching thirst by watching plums"
(comfort oneself by idle dreams)
Cao Cao of the Three Kingdoms period (220-265) was not only a capable politician in managing state affairs, but also a strategist good at leading troops in going to war.
One summer, Cao Cao was leading his troops in a punitive expedition against Zhang Xiu. It was extraordinarily hot. The burning sun was like a fire, and the sky was cloudless. The soldiers were walking on the winding mountain paths. The dense forest and the hot rocks exposed to the sun on both sides of the paths made the soldiers feel suffocated. By noontime the soldiers' clothes were wet through with sweat, and the marching speed slowed down. Some solders of weak physique even fainted on the roadside.
This story comes from "The Fake Tangery" in Anecdotes of This World by Liu Yiqing of the Southern Dynasties period (420-589). From this story, people have derived the set phrase "quenching thirst by watching plums" to refer to trying to comfort oneself by idle dreams.