Milk-first is a (dubiously) valid method, but it certainly doesn’t represent an optimal mixing strategy. It falls short on several counts. First and foremost, it fails to produce a consistent mixture and texture for cereal. As the milk is poured first, there is no cereal matrix awaiting moisture, and when the cereal is added it is too crunchy to consume comfortably. One may argue that pressure may be applied along the spoon to the floating heap of cereal, but as any experienced cereal-eater knows, it is difficult to push all floating flakes under the milk, and they bob up too frequently to absorb enough milk. Further – and more insidiously – the cereal/milk composition is not of an optimum ratio. As the cereal is not properly mixed with the milk, it has a higher buoyancy point, and it is difficult for the spoon to reach the milk and thereby scoop up some of the delightful nectar. Here, we assume that the eater desires to have a spoonful of milk below the cereal balanced on his spoon.
Cereal-first represents a far more advanced and consistent strategy. As the cereal provides a filtration matrix for the milk, the milk must flow through the cereal as it trickles to the bottom of the bowl. In so doing, it weighs down the cereal – and gives it a decent ration of milk to cereal. Nooks and crannies of flakes catch and hold milk, as an English-Muffin catches and provides texture for butter or other potential condiments. The accumulated milk weighs down the cereal. As any junior physics student could tell you, buoyancy is created when an object displaces more water than its overall density in water; that is, when its total volume in the water contains less mass than the water itself. With the extra weight of suspended milk pressing down on the bottom of our hypothetical cereal boat, it sinks further, making it easier for the spoon to reach the milk and achieve the ideal 70/30 milk/cereal ratio. Furthermore, a natural cycle is initiated, as submerged cereal becomes waterlogged and sinks to the bottom, where it cannot support the floating mass any longer, and the mass itself sinks, providing a slow-burn automated mixing process.