1. The Vertical Angle Theorem states that when two lines intersect, the opposite (or vertical) angles formed are equal. This means that if two lines cross each other, the angles that are across from each other at the intersection point have the same measure. 2. To understand why, consider two intersecting lines creating four angles. The pairs of opposite angles share a vertex and are formed by the same two lines. By the theorem, these opposite angles are congruent, which is a fundamental property used in geometry to solve many angle-related problems.