Bicycle crunches are a highly effective exercise for targeting the abdominal muscles. They are performed by lying on your back with your hands behind your head and your knees bent. Here's how to do bicycle crunches:

1. Start by lying flat on your back on a mat or the floor.
2. Place your hands behind your head, lightly supporting it with your fingers.
3. Bring your knees in towards your chest and lift your shoulder blades off the ground, but be careful not to pull on your neck.