A child from 3-6 years of age experiences a sensitive period for order. You can think of a sensitive period as if you were viewing a ballet and a spotlight is placed on one dancer. Your focus is brought to that dancer while the light shines on them and all else disappears into the darkness for a bit until the lighting is changed.
For the primary child the spotlight is focused on order. They notice everything and have a strong need for things to be in just the right place and to be able to predict what will happen next. You may notice a child becoming distraught if their schedule changes, a blanket isn’t arranged just right, or they can’t have the glass that they usually use. These things often seem small to adults but they are really disturbing to the young child who is trying to make sense of a world that always seems to be changing.