A child has fully understood a topic once they don’t require any guidance. This means that the zone of proximal development has been passed and the child no longer requires assistance when trying to do activities or answer questions. If the child is able to complete tasks on their own, they have reached a full understanding of the topic and could be considered as MKOs themselves. If a child is confident in a topic, you can place them next to a child who is less confident. They can socially interact with one another to help the less confident child pass through the zone of proximal development.
Source: Healthline, 2020, https://www.healthline.com/health/zone-of-proximal-development#educational-applications
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