There are many idiomatic expressions in Chinese language that the foreigners should know. 说梦话 and 除非太阳从西边出来 are two of them that the students often hear when they study Chinese. The literal meaning of 说梦话 is to talk in one‘s dream. However, the most important meaning that the students should know when they study Chinese is to indicate the words are not practical and can’t be realized. We can say 大白天说梦话. Here is a typical example 你说这项工作你一个人能完成，你这不是说梦话吗?

When we use 除非太阳从西边出来, we usually intend to express the things that are impossible to happen. Sometimes, students who are studying Chinese in China may hear 太阳从西边出来了. It means the rare things hard to come by. If someone who study Chinese doesn‘t like doing certain thing but one day you see he is doing, you can say 太阳从西边出来了.