Sitting Up: 6 to 7 months
All the floor work your baby has done up to this point is helping develop his back and abdominal muscles. He can now control his posture and stay upright -- although it may be a while before he can sit without support. For stability, your baby relies on his vestibular system, the body's mechanism for maintaining balance and sensing movement through space. Operating in the inner ears, this system receives information about the body's position and sends it to the brain so your baby can make adjustments in his posture and alignment to steady himself.
As your baby nears 8 months, he may reach across his body to grab food or toys. Moving the right hand into the left space of the body, and vice versa, uses both hemispheres of the brain, an important step in improving coordination.
Skill builder: Sit your baby down among some tempting objects. Place a few items at his sides so he works his midsection while he turns. Have cushions around, in case he's still wobbly. Rock and swing your baby, bounce him on your knee, or dance together to give his vestibular system a variety of sensations.