Playing Matchmaker

• If you shake up a bottle of vinaigrette-style salad dressing, the ingredients will combine briefly into a cloudy liquid, but then quickly separate back into oil and vinegar. Mayonnaise uses the same basic ingredients but remains combined, because of the lecithin and other emulsifiers in egg yolks. These are odd molecules, making a chemical bond at one end with anything water-based and at the other end with anything oil-based. Oil and vinegar usually separate, but these emulsifiers can help keep them together.

Drip, Drip, Drop

• Mayonnaise begins with its water-based ingredients. For homemade mayonnaise, those are usually egg yolks, vinegar or lemon juice, and a few drops of cold water whisked together. Next the oil is added, drop by drop, so it can slowly disperse into the water-based ingredients. However unlikely it seems, one egg yolk and a few drops of other liquids can easily emulsify a cup of oil. The crucial thing is to add the oil in a very thin stream -- almost a few drops at a time -- so the emulsifiers can help it bond to the water-based ingredients. If you go too quickly it will separate, but that can be fixed.

A Cold Shower

• If you're whisking mayonnaise by hand, or dribbling your oil slowly into a blender or food processor, you'll notice right away when the mayonnaise begins to separate. Suddenly, instead of the usual off-white gel, you'll see sloppy puddles of oil forming on the surface. That means you've got more oil than the water ingredients can cope with, so keep a glass of cold water beside you for those moments. If you add a few drops of cold water as soon as the mayo begins to separate, that will usually bring the oil and water back into balance and save your mayonnaise.

Step Up To the Yolk

• When cold water doesn't save the day, it's time to bring out the big gun. With mayonnaise, that comes in the form of another egg yolk. Whisk the yolk with a few drops of water in a second mixing bowl, then start incorporating the broken mayonnaise a few drops at a time. If it gets too thick, add a few more drops of water. Keep going until you've re-incorporated all the broken mayonnaise, and then add any remaining oil from the original recipe.