By law, margarine must have at least 80% fat, which is the same as butter. The remaining 20% is water. Some time ago companies that produced margarine started marketing a product with less fat and more water; these are called vegetable oil spreads. The more water a vegetable oil spread contains, the more likely cookies will spread. Unless the recipe is especially formulated for vegetable oil spreads, use butter (not whipped or light) or margarine (the package should contain the word "Margarine.") Do not use any margarines or butters sold in tubs.