Well, she knew the chemical formula, and that would probably be sufficient for Koschei, but not any kind of explanation.
"Well, on earth we have a lot of animals, mammals, they produce a liquid called milk to feed their babies, it's got lots of vitamins and things in that babies need, but adults do too, so humans drink a lot of cow's milk."
...which didn't sound very appetising, put like that. "And we make yoghurt from it, and cheese, and cream, and milkshake, which is like really creamy milk with fruit flavour. Or chocolate. That's my favourite."