Milk or cereals first

Pure physics.

Its just like any cold mixture when you're trying to fully immerse a dry solid with a liquid: Gotta add the solid first, as the subsequent addition of the liquid becomes the first mixing. Its dramatically easier than adding the solid on top of cold liquid.

Obvious difference is with heated liquid, it has to BE heated first, but the heat also makes it more receptive to mixing with a solid. Freeze dried coffee dumped onto near boiling water will damn near fully stir itself in within a second. With cold water, most of the freeze dried coffee can easily get caught on top of the water, blocked by slow dissolving, with streams of coffee solution that seep off the floating mass and collect at the bottom.... it may never fully integrate/mix depending on conditions, but it certainly takes longer than a second.

If the water is heated in something pourable, like a kettle, well then we are back to Solids First!

cereals

Cereal. Most of the time I eat it dry, but occasionally put some milk in.