Muscles work in pairs because muscle tissue can only pull (contract) and cannot push. To move a body part back and forth, one muscle of the pair contracts to pull the bone in one direction while the other muscle relaxes. Then the opposite muscle contracts to pull the bone back, allowing controlled movement at joints. These pairs of muscles are called antagonistic pairs, where one muscle is the agonist (contracting) and the other is the antagonist (relaxing) to create smooth and coordinated movement. For example, the biceps and triceps work as a pair to bend and straighten the elbow. Without muscles working in pairs, movement would be very limited or impossible because muscles cannot push bones back into position on their own.