The Tonic Neck infant reflex
is an automatic response that babies
have up until about four months of age. If you turn a child's head to the side while he is lying awake
on his back, he will reach one arm forward
in the direction his eyes are pointing, and the other arm will flex
Scientists don't really know why all newborns behave this way, but it may be a step in the development of the reaching and grasping motion.