Penguins are not mammals, because they belong to birds, and they have wings and feathers. Penguins will not secrete milk to feed their offspring, because mammals will secrete milk to feed their offspring. All mammals are viviparous except platypus and echidna. Penguins are birds, so penguins lay eggs to breed their offspring.

There are 18 species of penguins, mainly living in the southern hemisphere. Penguins are typical seabirds. Although they can't fly, their swimming skills are superior among birds.

Penguins swim at an amazing speed. Adult penguins swim at a speed of 20-30 kilometers per hour. Penguins feed on marine zooplankton, mainly Antarctic krill, and sometimes they prey on some brachiopods, squid and small fish.

The largest penguin is the Emperor Penguin, with an average height of about 1. 1 m and a weight of over 35 kg. The smallest penguin species is the Little Blue Penguin, with a height of 40 cm and a weight of 1 kg. It has its own unique structure, and its feather density is three to four times higher than that of birds of the same size.