Fortunately, we humans have a remarkable cognitive ability of abstraction and classification. The abstraction is to purposefully neglect special features of an object and emphasize the commonality between it and other similar objects. Based on this commonality, we classify these objects under a certain concept as a group. This ability helps us develop concepts such as "apple", "leaf", "tiger", and "jujube", and allows the calculation of sums among the same group of objects under certain circumstances. When we say "two apples", we actually abstract two objects that are classified as the same group into the same thing, eliminate the differences between them, conduct a "re-fungible" process, and then execute an operation of one plus one equals two, to create the concept of "two apples."