Child's Question on How Babies get in Tummy

Child's Question on How Babies get in Tummy

My stepson, who is 6 and is in kindergarten, asked me this morning how do babies get in the tummy. Since we were on the way to school, I told him we would talk about that after school. I don't want to say the Stork or it's magic but I also don't want to give him too much information either. He knows he and his half sister were in Mommy's tummy and came out. What is the best answer to give a curious mind?