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The magician walks onstage carrying an ordinary briefcase that he puts down on the table. When he opens it up, he removes a real bowling ball or basketball or another large item that cannot possibly fit in the briefcase. After the production, the case can be shown all around.
This is an improved version of the effect -- you can show both the inside and outside of the briefcase after the production.
Audiences are stunned! Bowling ball not included.