All reproduction requires the production of an egg cell. This fertilized egg then develops into a living organism. Some are retained within the mother's body, but the rest are expelled. Many have protective coverings called shells. Non-vertebrates typically lay eggs in water or damp areas. Those that do not produce a thickening around the egg to protect it from drying out. In the vertebrate animals, fish and amphibians lay unprotected eggs in water. They produce hundreds of eggs so that a few may survive to adulthood. Most reptiles produce a leathery-shelled egg that they bury. The earth absorbs sunlight and the warmth incubates the eggs of these cold-blooded animals. Birds lay a hard-shelled egg which is placed in a nest in most cases. Two mammals lay eggs (platypus and echidna). Their eggs are similar to reptile eggs, but are not buried. The hatchlings suckle milk from the mother.