These sauces are made by combining butter and flour and thinning with water or other liquid. A sauce should never be thickened by adding a mixture of flour and water, as in that case the flour is seldom well cooked; or by adding flour alone, as this way is certain to cause lumps. The flour should be allowed to cook before the liquid is added. All sauces containing butter and milk should be cooked in a double boiler. If so desired, any neutral oil–that is, vegetable or nut oil–may be substituted for the butter called for in the recipe. Care in preparation of a sauce is of as much importance as is the preparation of the dish the sauce garnishes.