Crunchs
Start by lying on the floor with hands behind head with elbows pointing outwards with legs bent and knees pointing to the ceiling and feet flat on the floor. Now slowly raise your head and upper body off the ground without using your hands to help in the movement, till a point where only the lower back and butt is in contact with the floor. Remain in this position for a count or two and then slowly fall back to the starting position. Repeat the movement as many times as you can.