In another advantage over the instant type, the homemade variety requires a lot less frantic stirring. Coaxing cocoa powder to dissolve into a liquid can be challenging, and often results in little particles floating on the top. This happens because cocoa powder is starchy, and when you hydrate a starch, it expands. Because dry cocoa particles tend to form clumps, as soon as you try to add milk or water, the outside edges of each chunk get wet, expand, and seal off the inside, leaving the inner powder dry. You can break up some of these lumps with your spoon, but when you're dealing with a cup full of liquid, some little buggers always evade your efforts. Heating the liquid helps, but often you'll still end up with tiny clumps that make your hot cocoa grainy.