1. The problem asks: The smallest angle of a triangle is always opposite the ___________ side. 2. In any triangle, the size of an angle is directly related to the length of the side opposite it. 3. The rule is: The smallest angle is opposite the shortest side, the largest angle is opposite the longest side. 4. This is because larger sides stretch the triangle more, creating larger opposite angles, and smaller sides create smaller opposite angles. 5. Therefore, the correct answer is "shortest". Final answer: The smallest angle of a triangle is always opposite the shortest side.