The assortment of knives should include a bread knife, butcher's knife, vegetable knife, a knife with a waved edge for cutting fresh bread and cake, and a spatula. The most durable knife is a hand-forged one in which the steel extends, flat and unnarrowed, to the end of the handle, and is fastened to it by rivets of steel, copper, or brass. Strength is lost if the steel extends only half the length of the handle. In the cheapest setting, known as the "twang," the steel is narrowed to a point, pushed into the handle, and fastened by adhesion. This is the kind of handle that is sure to come off at the most inconvenient time. Handles of beech or birch wood wear best. Rubber handles are unpractical because they shrink, swell, and burn. Ivory, pearl, or bone handles are likely to blacken or become loosened if they are put in water.