One of the most efficient exercises for abs.
Lay flat on your back with your hands behind your head.
Lift your legs and bend your knees so they are at a 45-degree angle, thighs are perpendicular to the floor, shins are parallel.
Start rotating your legs in a manner similar to riding a bicycle.
As you exhale lean up and turn to touch your right elbow to your left knee.
Then inhale and lay back down.
Exhale and repeat this movement to the left elbow, touching it to the right knee.
Do not press your chin to your chest.
The closer to the floor you keep your legs, the more load is on the target muscles.
You can start with an easier version, keeping your head and arms on the floor.