Mayonnaise and Hollandaise Sauce
These two egg-based sauces are examples of emulsions, among the most ethereal of the sauces in the classic canon. Emulsions are mixtures of two substances, such as oil and vinegar, that would not usually combine; made properly, they are perfectly smooth and delectably rich yet surprisingly light. Although other ingredients, such as butter and mustard, can act as emulsifiers, the egg yolk is the most effective. It is also a stabilizer, holding the emulsion over time.