A pervasive characteristic of development is that children’s functioning becomes increasingly complex—in language, social interaction, physical movement, problem solving, and virtually every other domain. Increased organization and memory capacity of the developing brain make it possible with age for children to combine simple routines into more complex strategies.88 The younger the child, the more she or he tends to think concretely and in the here and now. Yet in some ways, young children’s thinking can be quite abstract. For example, preschoolers know that adding always makes more and subtracting makes less, and they are able to grasp abstract ideas about counting objects such as the one-to-one principle.