The purpose of this study is comparing cognitive development of elementary 4^th graders and cognitive demand of activities in lesson ‘The mirror and the shadow.’ Also, the researchers tried to reveal how cognitive development of elementary 4^th graders and cognitive demand of textbook activities affects the achievement of students. To study students’ cognitive development, the researchers use GALT(Group Assessment of Logical Thinking). Piaget’s scientific thinking is used to evaluate cognitive demand of textbook activities. With several experts in science education the researchers discussed which scientific thinking is contained in each activities. The results were followed as : (1) The third of activities are significantly affected by students’ cognitive development. (2) The another third of activities are influenced by not only students’ cognitive development but also other factors experimental instrument. (3) The environmental factors had meaningful impact on the other third of activities.