Why are platypus know as mammals?

They feed their young milk produced by their body, that defines the mammal, reproduction isn't important in whether it is a mammal.

Further information:

The defining trait of a mammal is to have mammary glands, to produce milk for its young. The platypus produces its milk from numerous glands over its underside, unlike other mammals which have teats.

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.

Most mammals are also characterised by the following anatomical features, which are shared by the platypus:

- A flexible neck with seven cervical vertebrae

- Mammals also show enhanced neocortex development

- Sound is produced by the larynx (a modified region of the trachea)

- limbs are oriented vertically

- The mammalian heart has 4 chambers

- Internal temperature is generally high

- Egg development occurs in the uterus (excluding monotremata)

- They have sweat glands

- A single jaw bone

- Diaphragm

- Three bones for a middle ear

- Give birth to young alive

- Feeds milk to its young

- Has hair on its body