A birch (桦树) is a tree. It has a smooth bark (树皮) and is usually white. Many people like to drink birch sap. Sap is the juice inside the tree.
People make a hole in the tree. The sap comes out. It comes out very slowly. People usually leave a bottle there and then they leave. When they come back, the bottle is full. Now, there is an app (应用程序). The app sends a message to a person’s phone when the bottle is full.
People believe that birch sap is very healthy. But scientists say that it is not so special. Many people drink it because their parents and grandparents do it, too. People often use the same tree. The sap has no color. It is sweet when it is fresh. It gets sour after a few days.