Why are some topics in the Topic Carousel locked?

Topics are locked when students are not yet ready to learn them. They remain locked until the necessary prerequisite topics have been learned. For example, in order to learn addition of two-digit numbers with carry, students must first learn or have mastered addition of two-digit numbers. A lock icon appears in the topic card of the Topic Carousel to identify locked topics. Once all the prerequisite topics for a locked topic are learned, that topic is no longer locked and is now “Ready to Learn.” 
 
The Pie may show that 8 out of 10 topics have been learned for a particular pie slice, yet no remaining topics in that slice are available. This means the two remaining topics have prerequisite topics in other slices that must be learned first. Students should continue working in other slices, and eventually the two remaining topics in that slice will unlock.