What is the main reason that duck-billed platypuses are classified as mammals rather than birds? Consider that they possess a bill, as do birds, and lay eggs rather than give birth to live young like other mammals.

Platypuses are classified as monotremes, a group of mammals that lay eggs. Once the young hatch from their eggs, they cling to the belly of the mother and take in milk that oozes from openings in the mother's skin. This is the defining characteristic of mammals. Platypuses are also covered with fur rather than feathers, like other mammals.