Six-month-old babies often seem happy in their bodies.
They use all four limbs smoothly and rhythmically.
They enjoy physical movement for its own sake and keep testing the limits of their own strength as they struggle to roll right over or to lift their shoulders even farther from the floor.
They seem to understand, now, that all their different parts make up bodies that are all of a piece.
As we have seen, muscular control starts at the top and moves downward.
So at this stage the baby's use of his upper half, his head, shoulders, arms and hands, is well ahead of his use of his lower half.
He can use his arms and hands for accurate reaching out, and he can use his head to track moving objects with his eyes.
He does not yet have similar control over his hips, knees and feet.
It is mastery of these muscle groups for which the baby will now struggle.
The fight to stop lying around and become a sitter, a crawling quadruped and a walking biped is on.
If you put your six-month-old baby squarely on his bottom on the floor, spread his legs apart, get him balanced and then slowly remove your hands, he will probably stay "sitting" for three or four seconds.
His muscular control has already progressed downward to a point where he can hold himself straight from the top of his head to his bent hips.
But it has not yet reached a point where he can balance himself in this position.
By seven to eight months some babies will have solved this balance problem for themselves by leaning forward and supporting themselves with both hands flat on the floor in front of them.
If your baby takes up this position he will be comparatively stable and he will certainly be sitting, but it will not be a very useful king of "sitting alone.
" With both hands occupied in providing balance, he cannot play or even suck his thumb.
And because he has to lean forward to get his hands securely on the floor, he cannot see anything very interesting either.
Most babies will be eight to nine months old by the time they achieve independent balance, without support from an adult or their own hands.
But even now, sitting may be more for practice than for use.
Even when your baby can balance in a sitting position for a minute or more, he still topples over as soon as he turns his head or reaches out a hand.
It will take him another month of constant practice before sitting replaces lying down or being propped as the position in which he carried on most of his walking life.