Question

How do babies know , it is necessary to drink milk when they get hungry??? Even though their brain is underdevelopment?

Answer 1

they just naturally know it, like some of our instincts.

Answer 2

I'd say it's human instinct, or animal instinct. For both newborn babies and newborn animals alike, they seek out their mother's milk when they're hungry and afterwards, they catch on, and it probably becomes habit.