Grains refer to five kinds of grains. Books older than The Analects of Confucius, such as The Book of Songs and The Classic, have only "100 grains" but no "five grains". From 100 to 5 grains, did the variety of food crops decrease? That's not true. At the beginning, people often gave several different varieties of a crop a proper name one by one, so there were many lists. Moreover, the word "hundred" here is only used to refer to many meanings, and there is no real one hundred. The appearance of the word "five grains" shows that people have a clear concept of classification, and also reflects that there were five main food crops at that time. When the word "five grains" was first coined, it was not recorded what it meant. The earliest explanation we can see now was written by Han people. There are two main explanations of Han people and their descendants: one is rice, millet, millet, wheat and glutinous rice (that is, soybeans); Another way of saying it is marijuana, millet, millet, wheat and glutinous rice. The difference between these two statements is that there is rice without hemp and there is hemp without rice. Although hemp seed is edible, it is mainly used for textile. Grain refers to grain, and the former statement does not include hemp in grain, which is more reasonable. On the other hand, the economic and cultural center at that time was in the north, rice was a southern crop, and cultivation in the north was limited, so there might be hemp in the grain without rice. The crops mentioned in Historical Records, a book written by a historian, are wheat, millet, millet, glutinous rice and hemp, which belong to the latter statement. Probably for these reasons, Han people and people after Han people have two different interpretations of grain. Taken together, there are six main crops: rice, millet, millet, wheat, millet and hemp. There are four articles devoted to agriculture in the famous book Lu Chunqiu (written in the third century BC) in the Warring States period, among which the article Shenshi talks about the advantages and disadvantages of planting crops, such as millet, rice, hemp, rice and wheat. A grain is a millet. These six crops are exactly the same as the six mentioned above. These six crops are also mentioned in Lu Chunqiu. Obviously, the crops at that time were mainly rice, millet, millet, wheat, rice and hemp. The so-called grain refers to these crops, or five of these six crops. However, with the development of social economy and agricultural production, the concept of five grains is constantly evolving. Now the so-called grain is actually just a general term for food crops, or refers to food crops in general.