Great question! TTM is continually reviewing and improving content. These improvements are based on data showing how students are progressing through specific lessons. Sometimes the data tells us that a lesson needs to be modified or redesigned to improve student performance. In order to revise the lesson, we have to make it temporarily unavailable to teachers and students. Usually, we can do this when most students are not on the system. However, we don’t know when teachers will view a lesson so it can look like it “disappeared” to a teacher. We make every attempt to do this as quickly as possible and in a manner that does not interrupt the student or teacher experience. Again, most of these changes take place during evening and weekend hours and lessons are typically available again very quickly. If students encounter a lesson in their pathway that is temporarily unavailable, they will simply skip that lesson and come back to it once it is available. This is part of our continuing commitment to using data to improve results in every way possible.