Conservation, in the context of cognitive development, refers to the ability to understand that a certain property of an object remains the same, despite any transformation that has changed the appearance of the object. For example, children in the pre-operational stage of intellectual development often struggle to distinguish between reality and appearance. According to Piaget, they will fail to understand that if water is poured from a short and wide glass into a tall and thin glass that nothing has changed apart from the appearance. They do not understand that the quantity of water remains the same. This means that they make errors in the conservation tasks.