Birds, fish, most insects and most reptiles lay eggs, as do monotremes and amphibians. Birds lay eggs with a hard shell, and only one at a time, although they may lay several eggs to fill a nest. Reptiles like turtles and crocodiles lay eggs with leathery shells, and can lay hundreds of eggs at once. Fish and frogs lay eggs in the water, and most have no shells or a coating of jelly to make it hard for other things to eat them. Insects also lay eggs, and most are so small you can hardly see them. Only a very very small number of animals lay eggs. The platypus is one of them.