Skimming only reduces the fat content, it leaves the proteins and other soluble milk components. So you can dilute whole milk with skimmed milk to get semi-skim, but if you dilute it with water you just get weak flavorless milky water. While checking my recollection of the process I came across this amazing chart: LINK I wonder how many of these I can have with my breakfast today?

Isn't it more about the fat content (5%, 2%, etc)? I think to get skimmed milk you have to take full-fat milk and just lose some of the fat, not simply dilute it, or ou just get watery milk