A spiral curriculum has students repeat subjects at each grade level, allowing them to learn more advanced areas of the subjects until they have a complete understanding of the material. For instance, a child in first grade might learn the basic scientific fact that many plants use the sun's energy to generate their own food. In the second grade, this student might learn that the plant this process, photosynthesis, takes place in the plant's chloroplasts. At later grades, the student may learn to recognize different parts of the chloroplast organelle, including the outer membrane, granum, lamella, and stroma.
This approach to learning works well for many students because it lets them master basic concepts before moving on to more complicated details. In the example above, the student gets introduced to the most basic concept when she first enters school. After learning that concept, she is ready to acquire additional knowledge about the subject in later years. Eventually, students who keep up with their studies can amass large amounts of information by stacking new lessons on top of extremely basic concepts. This helps ensure mastery and allows educators to recognize a student's failure to absorb classroom materials before moving on to much more complicated ideas. A potential negative of this approach, though, is that students who fail to understand one year's material could face significant challenges in the following years because they don't have adequate mastery of the basic concept to understand more advanced, detailed material.