Sit-ups are a classic abdominal exercise that targets the rectus abdominis muscles (commonly known as the six-pack muscles) as well as the hip flexors. To perform a sit-up, lie on your back with your knees bent and feet flat on the floor. Place your hands behind your head or crossed over your chest. Engage your core muscles to lift your upper body off the ground towards your thighs, keeping your lower back in contact with the floor. Exhale as you come up and inhale as you lower back down with control. Avoid pulling on your neck and use your abdominal muscles to perform the movement. Sit-ups help strengthen the core, improve abdominal muscle endurance, and can be modified by adjusting the range of motion and adding resistance to increase difficulty and challenge different muscle groups.