Number (3): Everything in this space must add up to 3. The answer is 2-2, placed vertically; 1-0, placed vertically.
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The tree is rebuilt every frame. For scenes of this scale, quadtree construction is fast enough to rebuild from scratch each frame, though larger simulations may benefit from incremental updates. Each particle queries its neighborhood for potential collisions, typically checking only 5 to 15 candidates instead of all 40. Red highlights indicate colliding pairs.
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