Which of the following is a common method for frothing milk?

Using a steam wand is a common method for frothing milk because it introduces steam into the milk, creating a creamy texture and the necessary microfoam. This process not only aerates the milk but also heats it to the desired temperature, resulting in a velvety consistency that is ideal for lattes and cappuccinos. The steam wand works by forcing steam through the milk, which bubbles and incorporates air, enhancing the overall flavor and mouthfeel.

In contrast, boiling the milk can lead to a scorched taste and does not produce the desired frothiness. Shaking milk in a jar can create some foam, but it lacks the finesse and temperature control provided by a steam wand. Mixing milk with cold water would dilute the milk and negate its ability to froth properly. Thus, of all the methods listed, using a steam wand is by far the most effective and widely accepted technique for achieving beautifully frothed milk.