Q: Whenever I try to make mayonnaise, I always wind up with a soupy mess. What is the real process of making it and why can't I do it?

A: Mayonnaise, hollandaise, vinaigrettes and all their variations are emulsions. An emulsion is when water and fat come together to make a creamy thick liquid, generally a sauce. Water and oil, as the expression goes, don't mix. However, when liquid fats are slowly incorporated into watery liquid, the fat molecules disperse in the liquid and result in a thick and creamy suspension. This process, fat slowly whisked into a thin liquid, is repeated constantly in cooking: oil into vinegar for vinaigrettes, oil into egg yolks with lemon juice for mayonnaises and hollandaises, and chilled butter into wine for beurre blanc.

Start with the liquid (usually an acid like lemon juice, vinegar or wine) and the seasonings (herbs, mustards, salt, pepper, etc.) in a bowl. Blend the liquid and the flavorings with a whisk. Make sure that the bowl is stabilized with a dishtowel underneath it so you can use one hand to whisk and the other to pour while making the emulsion. Then, in a thin stream, while whisking, start drizzling in the fat (usually melted butter or oil). The mixture will at first be cloudy, then it will thicken. If it is not thickening, stop pouring in oil. Whisk in one corner of the mixture, coaxing part of the oil and acid into an emulsion. Then widen the amount being whisked to incorporate the rest of the oil.

When an emulsion breaks down, the fat and liquid separate, and looks curdled. The standard kitchen phrase for this is that "the sauce is broken." Two key techniques that almost always ensure a successful emulsion are, first, a stable temperature, making the emulsion at neither too hot nor cold a temperature, and second, always add the fat slowly into the watery liquid. Certain foods help to make tighter, thicker emulsions of fat and water, for example mustard, cream and egg yolks in vinaigrettes.

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