Now and then everyone has the urge to whip up a batch of concrete. A strong and practical building material, concrete will last for a long time. It has many uses around the home. A concrete path will keep children from wearing out the ground. A concrete patio will keep guests outside where they belong. A ten-foot wall with spikes will control the new neighbors. To make concrete walls, sidewalks, planters- anything with a definite shape- you will need to build a form first. The form is made of wood and form building is governed by a universal law that says: "No matter how strong you make the form it won't be strong enough and wet concrete will ooze out and spread all over everything and you will be helpless no matter how much you yell and run around." The law is hard to believe and hard to memorize, but do both. The larger the project, the more dramatically the law will be demonstrated. Say you plan to make a concrete swimming pool in your back yard under the shade of the old washline. First you dig a pit with vertical sides. Then you build forms, braced and crossbraced, designed to shape the inside walls and contain the concrete mix while it hardens. The ready mix men arrive with tears in their eyes and after looking over your setup and surviving a coughing fit, they assemble sections of metal trough leading from the truck to the pool. Everything goes well at first then, suddenly, you hear a sound of bulging plywood. Then, with the crack of wood and the pop of nails and the snap of braces, concrete flows out from under the forms with a tired sigh. The delivery men put away the trough, paint another little homeowner on their truck door and drive away. Eventually the whole fills with garbage, lawn refuse, and dirt till it is again ground level and another garden is born.