RuneScape Classic Wiki
RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Milk
The Old School RuneScape Wiki also has an article on Milk.

Milk can be gained by using a bucket on a cow. It is used in the Cook's Assistant quest. It is also used to make cakes and chocolaty milks. Milk can be purchased from Hudo Glenfad's groceries in the Grand Tree.