Egg yolks not only thicken but also enrich a sauce. Never add them directly to hot liquid. Stir them into a little cream, then incorporate with them some of the hot sauce in preparation. Stir this mixture into the remainder of the hot liquid and continue to stir over low heat until the sauce thickens. Do not allow the sauce to boil, or it will curdle. If this happens, plunge the pot into cold water and stir; or beat in a small amount of chilled cream. It is generally safer to add egg yolks to a mixture in a double boiler over, not in, boiling water, unless you can control the heat source very exactly. Two or three egg yolks with a little cram will thicken 1 cup of liquid. Egg yolks added very slowly to melted butter or oil with constant stirring will produce an emulsion which is quite thick. Suitably seasoned, this becomes the base for holandaise or mayonnaise.