There's the proverb that oil and water don't mix, an axiom vividly illustrated by a bottle of vinaigrette. It forms a cloudy mixture when shaken, but quickly separates into distinct layers of oil and water-based vinegar. Mayonnaise takes those same ingredients and brings them together in a stable emulsion by incorporating egg yolks into the process. The eggs form chemical bonds with both the oil and the water-based eggs and vinegar, enabling fine droplets of oil to disperse throughout the eggs and vinegar. When it's done right, the mixture becomes very stiff and spreadable.

Mayonnaise recipes start with one or more egg yolks in your mixing bowl. One will emulsify a cup of oil though many recipes call for two. Whisk the yolks with a small pinch of salt, which helps them thicken, and then begin adding the oil a few drops at a time. Whisk until the oil is completely incorporated before adding a few drops more. Once the egg yolk has about doubled in volume, you can begin adding oil in a very thin, fine stream. It should blend into the mixture as quickly as you pour it, so slow down or stop for a moment if that isn't happening.

The consistency of your finished mayonnaise is affected by the ratio of water-based ingredients to oil. The more water it contains the thinner it becomes. Aside from the moisture of the egg yolks, most of the water comes in the form of lemon juice or vinegar. Some recipes call for it to be whisked into the eggs, but your mayonnaise will thicken better if you add it in the late stages. If you find that your mayonnaise has a smooth but thin, sauce-like consistency, add more oil -- a tablespoon at a time -- and keep whisking. It will thicken naturally as the additional oil disperses through the emulsion.

If your mayonnaise is thin and streaky with visible ribbons of oil and mayonnaise it has curdled or 'broken." That's dispiriting, but perfectly fixable. Keep a small cup of ice water next to your work area and at the first sign that your mayonnaise has broken add a few drops of cold water. This should bring the emulsion back together enabling you to continue. If it doesn't, you'll need a second mixing bowl. Whisk a fresh egg yolk and a pinch of salt in the new bowl and start pouring in the failed mayonnaise a few drops at a time. Work slowly and carefully until it's mixed in then add any remaining oil from the original recipe.