Of course the simplest way to straighten a warped board is to put a weight on it, but the difficulty here is that it usually will stay straight only while the weight is on it, unless you leave it longer than the patience of the average amateur lasts. To do this (or to warp a straight board either) with some chance of success, (1) heat one side, or (2) wet one side, or (3) wet one side and heat the other, or (4) wet both sides and expose one to the fire (Fig. 687). But do not be too sure that the result will be lasting. Sometimes it will and sometimes not. Another way is to thoroughly soak the board, press it into shape between clamps or under a weight, and leave it until dry; a week or more is none too long, and boiling water is better than cold. Simply laying a board down on a flat surface will often cause it to warp, because the two sides of the board will be unequally exposed to the action of the atmosphere. Planing off one side only, or planing one side more than the other, often produces the same effect. See pages 50-53 and Appendix.