The digital learning object L7803 Finding symmetry: one line: city introduces the idea of line of symmetry. Testing whether a shape 'folds' entails a very basic understanding of proof: the fold line is true because it matches.
The online activity gives young students a chance to test whether presented shapes have a line of symmetry or not. It provides a tool for students to answer and discuss the question of "How do you know?" The reasoning involves ideas such as
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same or different
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true or not true
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visualisation
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If… then…
These are all introduced in Big Ideas.