Grab your whisk and stir the milk, slowly at first. Increase your stirring speed to that of beating eggs as the milk warms (be careful not to splash yourself with the hot milk). A bubbly foam will begin to appear on the top of the milk. Like the steaming handle on an espresso machine, the whisk aerates the liquid, filling it with bubbles that make it puff up and thicken. The first bubbles that form on the top will be wide and loose, but the more you whisk, the smaller and more condensed the foam will become. Whisk until you have achieved the desired amount of foam, being careful not to let the milk boil, and remove from the flame. You want the milk to be hot, but not that hot, and letting the milk boil ruins the taste of the milk and sometimes reverses the foaming. If you think the milk is about to boil, turn off the flame and continue whisking.