We poured 4-ounce samples of both nonfat and whole milk into oversized cups, whipped them up with a battery-operated milk frother, and compared the results. The skim milk [top] indeed frothed better than the whole milk [below]: The foam was thicker, more stable, and composed of finer bubbles; it rose up higher in the cup. The whole milk foam took longer to form and was made of larger bubbles that deflated in a matter of minutes. As it turns out, milk fat contains monoglycerides and diglycerides, two substances that happen to be good emulsifiers but have a negative effect on the formation of foam. They destabilize the air bubbles introduced into milk when foam is made, thereby impeding the foam’s formation. Skim milk contains a much smaller amount of these emulsifiers, thus making frothing easier. Although most baristas wouldn’t recommend using skim milk to make rich and creamy cappuccinos, if plenty of airy foam is your goal, the less fat in the milk, the better your results will be.