Where the babies come from?

This is the moment I have been trying to avoid. Alec and I were looking at his friend's Dasha' blog. She drew a beautiful family picture and we talked about how good she drew...

Alec: "So, that's her Mom, her Dad, kak Dasha and her brother?"
Mom: "That's her sister, sweetie"
Alec: "She doesn't have any brother?"
Mom: "No. Just like you, you have a baby sister, too."
Alec: "I want a baby brother, Mom"
Mom: "Uh...you do, huh?"
Alec: "Yes. How do we get it?"
Mom: "Well, its from my belly"
Alec: "If its inside your belly, how did you make the baby, Mommy?"
Mom: "Let's wait for Daddy to come home and we ask the question together. Because I don't know how to explain it to you."
Alec: "No, you can do it. Just use your imagination"