Horizontal migration refers to the interaction between experiences at the same level of abstraction and generalization, and is the interaction between two experiences that belong to the same level of level in difficulty and complexity; vertical migration refers to interactions at different levels of abstraction and generalization. The interaction between experiences is the interaction between a higher-level experience with a higher level of generalization and a lower-level experience with a lower level of generalization. According to the above definitions, the following are vertical migration:. ['Elementary school students will learn to write "Sen" after learning to write the word "Wood".', 'After learning the concepts of "plant" and "animal", it is helpful to learn the concept of "biology".', 'Learning concepts like "tiger" and "lion" can help identify unfamiliar whales or dolphins.', 'After learning the knowledge about squares, it helps to learn about rhombuses, rectangles and other related knowledge.']