Tension in a string pulled by equal but opposite forces is not zero, despite the net force being zero, due to the interactions at both ends of the string. According to Newton's third law, the forces acting on the string from both ends are not action-reaction pairs because they act on different bodies. When equal forces are applied, the string experiences tension as it stretches, even though its center of mass does not accelerate. In elementary physics, strings are often considered massless, leading to the assumption that tension is constant and equal to the applied forces. Thus, applying equal forces at different points can indeed create tension in the string.