Discrepant events are the cornerstone of a constructivist science education. A good demo will force a student to confront their preconceived notion of how a phenomena works. Students must work at trying to resolve the conflict between what they just saw and their prior knowledge. Magic tricks are a perfect example of that. Students think what they just saw is magic or they try to figure out how the magic trick works. This is a simple magic trick that involves the scientific concept of friction (and a little bit of tension).
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Response: Learn MagicScienceFix - Science Fix - Using Magic as a Discrepant Event