A platypus lays eggs but feeds hatchlings milk without nipples. What makes that less contradictory?

Answer: Milk defines the mammal

Nipples define nursing — Nipples are common in mammals, but they are not the core definition of nursing. Platypus mothers lack nipples and instead release milk onto abdominal patches where young lap it up. The useful correction is that the milk-producing gland matters more than the particular delivery hardware.

Milk defines the mammal ✓ — Correct. Platypuses preserve an older-looking mix: they lay eggs, yet they still use mammary glands and milk genes shared across mammals. Genome work reports casein genes for milk alongside a retained yolk-related vitellogenin gene. So the animal is not half-bird; it is a mammal showing that milk and egg-laying once coexisted in our deeper family history.

Live birth defines mammals — Live birth is common among mammals, but it is not universal. Monotremes such as platypuses lay eggs and still qualify as mammals because they produce milk. The counterexample is the whole point: a platypus breaks the shortcut people often use for mammals while keeping the deeper mammalian trait.