Comment by tialaramex

Ordinarily in Minecraft if you click a cow with a bucket it fills with milk, a simplistic model of how this works that reflects probably the level of thought most people have about where cow milk comes from.

One of the mods (Animalia maybe?) obliges you to do this properly. Find a male and a female, enclose them and make sure they're well fed so that they'll breed, wait a while for the female to give birth, steal the calf, and then you can collect her milk. I would bet far more players in games with that mod go vegan because it was a lot of hassle to make these arrangements compared to making soy milk.