Cow's milk contains calcium because cows eat grass. Calcium is a mineral that comes from the ground, which means, like grass, all green leafy vegetables are teeming with this nutrient. Let's explore the rationale behind human adults drinking another animal's milk when we don't even drink our own species' milk into adulthood. Perhaps all the calves are laughing at us, for even they stop drinking their own mothers' milk when they become adults. In other words, I think we have a lot to learn from the cows.