Sit your child astride on a roll or over your leg.
Make sure that the feet are firmly on the floor, with the weight evenly on both feet.
You can support the child's position from the waist, hips, legs, or feet.
Encourage the child to reach for a toy from the side so that one side of their trunk contracts and the other stretches.
And, as the child's skills develop, you can encourage them to reach for toys from different directions, heights, and distances.
