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 have 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 some develop within their mothers (such as mammals), almost eliminating the need for eggs. Most eggs are considered tasty, such as chicken eggs, 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.