Babies sometimes seem like human rubber bands because of their bone structure, says Jennifer Shu, M.D., a pediatrician in Atlanta and coauthor of Heading Home With Your Newborn. Infants have more than 300 bones that will eventually fuse as they age (adults have only 206). Until then, the spaces between the bones are filled by pliable cartilage, like what's in the tip of our noses. This comes in handy in utero, when babies' bodies have to fit comfortably in such a tiny space; the ability to perform circus-worthy contortions is just a side benefit.
As your baby grows, her bones will strengthen and this flexible phase will end -- though you'll probably be too busy chasing her around the house to miss it!