Video narration
If you cut a ball in half, what makes that flat slice a plane of symmetry? In a flat drawing, you look for matching halves; here, you compare whole sides of the ball. Imagine a smooth ball with an invisible flat surface passing through its center. This plane runs through the ball, not just along its outer skin. It divides the three-dimensional ball into two parts. The halves match as mirror images: reflect one across the surface, and it fits the other. Now move the surface off center. You still get two pieces, but they no longer match as mirror images. That's the key: splitting alone isn't symmetry. For the halves to count, each must be the reflected partner of the other. That flat dividing surface is what we call a plane of symmetry. So what makes the center cut special? It divides the ball into two mirror-image halves.