A hen's egg takes twenty-one days to become a chick. Before it comes out, the chick breaks into the air space inside the egg and starts to breathe air, and it may peep from inside the shell. It also grows a small, sharp egg tooth at the tip of its beak, which it uses to peck through the shell. A newly hatched chick can live on the yolk it took in before hatching. It does not need to be fed for the first few days.

Most frogs come out of eggs as tadpoles. Tadpoles have gills and a tail. As a tadpole changes into a frog, its hind legs sprout first and its front legs come after, and it grows lungs and loses its gills, so it can breathe air. A young frog still has a stump of a tail when it first comes out of the water.

A monarch caterpillar eats only one kind of plant, milkweed. A growing caterpillar outgrows its skin, so it sheds the old skin and grows a new one, which is called molting. It goes through five growth stages, and each stage between molts is called an instar. A full-grown caterpillar hangs upside down in a J shape and sheds its skin one last time, and under it is the green chrysalis.

A new butterfly comes out of the chrysalis with crumpled wings, and it must pump fluid into them and let them dry before it can fly. Going through all four life stages, egg, larva, pupa and adult, is called complete metamorphosis.

A newborn kangaroo is only about the size of a jellybean, yet it crawls into its mother's pouch by itself, using its front legs. A baby kangaroo is called a joey. As it grows, it starts to hop out and explore, going farther and for longer each month, and it hops back in to stay safe. It keeps drinking its mother's milk until it is about 10 to 12 months old.

A newborn giant panda is pink and blind, and it weighs only about 1/900th as much as its mother. It is about the size of a stick of butter. The cub cannot move around much until it is about three months old, and by three to five months it walks pretty well. At five to six months it starts to try eating bamboo, but it keeps drinking its mother's milk long after that.

Some babies are ready almost at once. A giraffe mother gives birth standing up, so her newborn calf falls to the ground. The calf is about six feet tall, taller than most grown-up people, and it is on its feet within an hour of being born, and often running. A foal, which is a baby horse, can stand up within about an hour of being born, and it can walk, and even run, soon after.

An emperor penguin dad keeps the egg warm on top of his feet, tucked under a fold of belly skin, while the moms go to sea to feed on fish. For the first weeks, the chick stays on its parent's feet under a warm flap of belly skin. A mother sea turtle, by contrast, does not stay to care for her eggs. She goes back to the sea, and by the time the hatchlings come out of the nest at night, she is gone.

Hatchlings find the sea by the slope of the beach and by the moon or starlight shining on the water. Bright lights from people can confuse them. After they reach the water, young sea turtles swim out to the open ocean, where they live for several years. Many years later, a grown female usually returns to lay her own eggs on a beach near where she hatched.

Words to know

• chrysalis: butterfly's resting case
• molting: shedding skin
• metamorphosis: big body change
• instar: growth stage
• yolk: egg food