When baby Rex is bounced gently on his father's knee, his father can see the skin on the top of Rex's head move up and down, sort of like the skin is covering a bowl of jello. This is because:

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At the time of birth, the soft, bony plates of the skull-called fontanels-are connected only by cartilage areas, which are squeezed together in the birth canal to allow the baby's head to pass through, giving the newborn a "cone-head" appearance. Fontanels do not fully harden and fuse until late in infancy, which is why an infant's or young toddler's head must be protected from bumps; otherwise, there is a risk of concussion.

When Baby Rex Is Bounced Gently on His Father's Knee