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Children often enjoy putting 'like' things together, when putting things away and gathering them together.
Adults could have lots of boxes (or trays), perhaps 'shoe-box' style, available for children to use as well as large groups of small items like zoo/farm animals, train set carriages or any similarly shaped items.
Encouraging mathematical thinking and reasoning:
The Mathematical Journey
Properties of shapes:
Same and different:
Counting and cardinality:
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