1. **Problem:** Explain why opposite angles in a parallelogram are always congruent. 2. **Definition and properties:** A parallelogram is a quadrilateral with both pairs of opposite sides parallel. 3. **Key property:** Opposite angles in a parallelogram are congruent, meaning they have equal measure. 4. **Reasoning:** Consider parallelogram ABCD with angles $\angle A$, $\angle B$, $\angle C$, and $\angle D$. 5. Since AB is parallel to DC and AD is a transversal, alternate interior angles $\angle A$ and $\angle C$ are congruent: $$\angle A = \angle C$$ 6. Similarly, since AD is parallel to BC and AB is a transversal, alternate interior angles $\angle B$ and $\angle D$ are congruent: $$\angle B = \angle D$$ 7. **Conclusion:** Opposite angles in a parallelogram are congruent because they are alternate interior angles formed by parallel lines and a transversal. This property holds for all parallelograms.