vAlthough Hannah understands the principle of conservation and basic mathematical operations, she has difficulty understanding hypothetical questions and attempts to solve problems unsystematically, by trial and error. According to Piaget, Hannah has not yet attained the cognitive skills characteristic of the stage. Although Hannah understands the principle of conservation and basic mathematical operations, she has difficulty understanding hypothetical questions and attempts to solve problems unsystematically, by trial and error. According to Piaget, Hannah has not yet attained the cognitive skills characteristic of the stage. Although Hannah understands the principle of conservation and basic mathematical operations, she has difficulty understanding hypothetical questions and attempts to solve problems unsystematically, by trial and error. According to Piaget, Hannah has not yet attained the cognitive skills characteristic of the stage. Although Hannah understands the principle of conservation and basic mathematical operations, she has difficulty understanding hypothetical questions and attempts to solve problems unsystematically, by trial and error. According to Piaget, Hannah has not yet attained the cognitive skills characteristic of the stage.