This chapter calls for a little bit of scientific background. An emulsion is the mixture of two liquid ingredients that normally don’t mix, held together by a third ingredient that stabilizes the mixture and prevents it from separating. Two liquids can form an emulsion by being vigorously shaken together—like oil and vinegar in a jar—but the emulsion will not hold, and the two will eventually separate. That is something we call a temporary emulsion, not to be scorned! But adding an emulsifying agent like egg yolk, egg white, flour, lemon juice, or mustard will create a thick and velvety sauce that holds together permanently, such as mayonnaise, aïoli, vinaigrettes, and roux-based sauces. Understanding how emulsions hold and stay together can help us make better sauces every time and help prevent the emulsion from breaking—or allow us to fix it easily when it does. Emulsions can be delicate, but a split emulsion is both avoidable and repairable. See the tips under each sauce or condiment recipe for how to avoid or fix a broken emulsion.