The twin acrobats rehearsed their mirror routine, starting on opposite sides of the stage — Lina always in red on the left, Mara in blue on the right. Each night they stepped one pace closer to center. But midway through the Wednesday show, they pulled the switch they had been practicing in secret: Lina slipped into blue and crossed to the right, Mara into red and crossed to the left. When they resumed on Thursday, the audience saw red on the right and blue on the left — the mirror image of opening night — and no one in the crowd could say when it had changed.