Milk

The Milk is a Rare item in Golden Sun: The Lost Age, and part of the trading sequence that will allow you access to the optional dungeon Islet Cave in the Sea of Time Islet. It is in the possession of the cow at N Osenia Islet, and it will give it to you if you give it the Red Cloth from SE Angara Islet. The Milk is then traded for the Lil' Turtle at West Indra Islet.