Talking about mammals

Conversation I was having with the boy today:

Me: "So what makes a mammal a mammal?"
Ben: "They make milk for their babies."
Me: "And?"
Ben: "And they feed that milk to their babies."
Me: "And they give birth to......"
Ben: "And they give birth to alive babies."
Me:" As apposed to?"
Ben: "Dead babies."
Me: "Um no."
Ben:" No? What then?"
Me: "Eggs."
Ben: "Oh."