Conservation is one of Jean Piaget‘s noted developmental accomplishments for children. It’s when a child can grasp the concept of an object or substance not changing its volume or amount. Usually, this development is observed in kids between the ages of 7 and 11, or probably 3 months if your name was Lex Luthor.
An example would be a child seeing three big bowls of hot steaming oatmeal in a bear’s house. While one ball is very tall and skinny, the other two are flatter and much wider, even though all of them have the same volume. A child with a lack of conservation would choose the tall skinny glass because they think it has more oatmeal inside it. The bears would not be pleased with such an outcome and possibly eat the child after remembering they don’t eat oatmeal in the wild.