Final answer:
Muscles occur in pairs and work together to move bones by contracting and pulling on them.
Step-by-step explanation:
Muscles can only contract, so they occur in pairs. For example, in the arm, the biceps muscle is a flexor that closes the limb, while the triceps muscle is an extensor that opens the limb. This configuration is typical of skeletal muscles in humans and other vertebrates. Most skeletal muscles exert much larger forces within the body than the limbs apply to the outside world, thanks to their attachments to bones and joints via tendons. When muscles contract, they pull on bones and allow the body to move.