An egg is a reproductive capsule produced by many animals to reproduce. Fertilized eggs contain embryos which develop into whichever animal laid it.Some animals lay eggs which are only visible at a cellular level, like humans for example. Usually the egg contains all the nutrients the developing animal needs, but mammals for example develop within their mothers, eliminating the need for eggs. Most eggs are considered tasty, such as chicken eggs by humans, but they tend to go bad after a short amount of time unless kept cold. However, eggs provide excellent sources of nutrition because of the nutrients they contain.