Crunches are a popular abdominal exercise designed to strengthen the core muscles. To perform a crunch, 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, then lift your shoulders off the ground while engaging your abdominal muscles. This movement helps target the rectus abdominis, the muscle responsible for the "six-pack" appearance. Crunches can be modified to increase or decrease difficulty. For example, bicycle crunches involve alternating knee lifts and elbow touches, while reverse crunches focus on lifting the hips off the ground. Regularly incorporating crunches into a fitness routine can improve core strength and stability.