How do people stay warm?!

Today in the car, Jack spotted some geese flying in the air.
Here is the conversation that transpired:

J: "Look at the birds, Mommy!"
Me: "I bet they are cold up there."
J: "No, they aren't. Birds and gooses and ducks have feathers to stay warm." "And cows have fur, so they don't get cold either."
Me: "Well, how do people stay warm then?"
J: "The heater."

I laughed for a while after that!