"All right everyone, it's time to clean up and move to your next center!"
This announcement used to make me die a little bit inside every time I heard it in another teacher's classroom. Children hung their incomplete paintings and took apart 80 pieces of their100-piece puzzles. Unfinished Lego constructions, concepts not quite mastered and books closed midway through all went back to the bins and shelves, and children moved on to partially complete another activity.
There is a reason the 3-hour work period is sacred in a Montessori classroom. Visitors are instructed not to intervene in the learning process, either by asking questions or pointing out a child's errors. Children work until they are finished. They often figure out their own mistakes in the process. Self-correction is a sign of deep learning. Mastery of a new concept sometimes takes 10 minutes and sometimes 90. The children know when they are finished.
Enjoy this 5-minute time lapse video of a preschool Montessori student managing his own 3-hour work cycle at Dundas Valley Montessori School in Ontario.