Almost all vertebrates reproduce sexually and all take care for their offspring in different ways. Some, including lizards, fish, and amphibians sometimes develop from unfertilized eggs. Birds use internal fertilization to reproduce. They lay eggs. This suits their environment because they need to fly, and to fly efficiently they need to have as little weight as possible. The bird's eggs are similar to reptiles. They have a hard outside shell. Most birds incubate their eggs until they hatch. This period could be from thirteen to fifty days. When a bird is ready to hatch it uses its beak to poke a hole and prod itself out of its shell. Then it collapses until its feathers dry out. Some birds can take care of themselves after a couple of hours, but most depend on their parents for care and food for days or even weeks. Most amphibians reproduce externally, but some have internal fertilization. The females can release as many as 200 eggs at a time, which the male fertilizes. This suits their environment because it is aquatic and the eggs that the females release into the water can attach to underwater plants. Their eggs can then get nourishment and be protected by these plants. After one to three weeks the eggs hatch into tadpoles. This type of reproduction strategy is the main type in amphibians. Other types of strategies are mainly adaptations to a lack of a suitable water environment to lay the eggs. When this occurs, some adaptations may include protecting the eggs in the mouth or having a pouch on the back where the tadpoles can cling to, to keep moist. These adaptations are made by the parents to protect and care their young and to make sure they have offspring. There are three different reproductive strategies for mammals. One type is the egg-laying mammal, which is the platypus and its relatives. The platypus is like a reptile in the sense that it lays eggs, but it is like a mammal because after the young are hatched they feed on milk from the mother.