There is such a concrete explanation in the second book of fifth grade mathematics. Volume calculation formula is a volume calculation formula, which is different from volume. Volume measures its length, width and height from the inside, and volume measures its length, width and height from the outside. Because the volume is the size of the inner space of the container, the middle of the cuboid is still a cuboid. What is the volume inside? Calculate the volume inside, which is the volume of this rectangular container. So we still use the volume calculation formula.