ANSWER: Believe it or not, platypuses are not the only mammals that lay eggs. Mammals that lay eggs are called monotremes and include platypuses and echidnas, both of which live in Australia. Like all mammals, monotremes are warm-blooded, covered with fur and nurse their young. Animals are so interesting because they don’t always follow the rules. Some mammals lay eggs; some snakes give birth to live young. read more
Now only two kinds of monotremes are left on the planet — the duck-billed platypuses and the four species of echidnas, or spiny anteaters. Like all mammals, they possess hair, milk, sweat glands, three middle ear bones and a brain region known as the neocortex. read more
There are other reasons why platypuses are classified as mammals, such as having skin, hair or fur, being warm blooded and breathing via lungs (not gills). It just happens that platypuses, like echidnas, are egg-laying mammals, or monotremes. But they are still mammals. read more