Most female animals lay eggs. Inside the egg is a food supply called the yolk and a very early form of a new animal. This form is called an embryo. The embryo and yolk are protected by an egg case. In reptiles the egg case is leathery. In birds it is made of shell. After the egg is laid the embryo starts to grow. It uses the energy and material in the yolk. When the embryo is fully formed it breaks out of the egg. This process is called hatching.

Mammals are unusual because most of them do not lay eggs. The embryos (unborn babies) grow inside the bodies of the females. Just two mammals lay eggs. They are the duck-billed platypus and the spiny anteater.

Eggs are a very good source of protein and fat as well as vitamins. Eggs also last a long time before going bad and they don't always have to be kept in a refrigerator. Chickens, and other large egg-laying birds, are also easy to keep and will scratch around for their own food if necessary. Eggs are also a very good food for people who have been unwell, because they will not upset your stomach. This is why eggs have been used as a source of food for so long. The only thing eggs don't have is very much fibre.