Quenching thirst by thinking about plums
望梅止渴    wàng méi zhǐ kě lit. to quench one"s thirst by thinking of plums (idiom), fig. to console oneself with illusions
The expression is a chengyu (成语, cheng2yu3).

There's a story behind it: in the Three Kingdoms period, general Cao Cao (曹操    Cáo Cāo (155-220)) was leading his troops on a journey to battle. They were in a dry place and his troops were very thirsty. Cao Cao tricked them by telling them they were not far from a wood of plum trees, where the plums were juicy and sweet; they would be able to quench their thirst there. Thinking of the plums got the soldiers' mouths watering. They no longer felt so thirsty and were able to continue their route. Soon after they arrived at a place where they found water.

I got this info from a very good French-Chinese book called "Il était une fois Au pays des chengyu" by Fei-Fei CHEN and Claire JOURNIAC.