Infants and young children may suck on thumbs, fingers, pacifiers or other objects. Sucking makes them feel secure and happy, and helps them learn about the world around them.
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Sucking is a reflex common to all mammals and is present at birth. Many babies have even been observed sucking their hands or fingers while still in utero. In babies, sucking is a primitive or newborn reflex. Primitive reflexes originating in the central nervous system and are exhibited by normal infants but not neurologically intact adults. These reflexes disappear or as a child moves through normal stages of development
Many primitive reflexes have a survival value. For example, a baby will instinctively suck at anything that touches the lips or the roof of their mouth. This burst of sucking simulates the natural eat process by pressing the nipple between the tongue and palate to draw out the milk and moving the tongue to coax milk from the nipple to be swallowed by the infant. In addition sucking can encourage a parent to respond lovingly and feed more competently.
The difficulty lies in that this reflex response does not differentiate sources of nourishment from other substances. Therefore infants suck on everything that touches their lips or mouth.