Have you ever built something? Let's say take shop at school, and your project is to construct a wooden birdhouse. You start with raw materials -- wood, nails, glue and paint -- and you go to work.

As you build, you saw off scraps of wood, and make sawdust. You bend a couple of nails and throw them aside. You use an old rag to wipe off a little extra paint. When your project is finished, you pick up all the bits of leftover material and toss them away. Those scraps of lumber and dirty rags are the waste produced as you use energy and materials to build the birdhouse.

Almost any job -- whether it's making scrambled eggs, building a barn or doing your math homework -- creates waste products. When the task is done, you have to get rid of the eggshells, the leftover lumber, or the balled-up pieces of scratch paper.

The job of keeping the human body going creates waste products too. These products are chemicals your cells give off as they burn energy. To stay healthy, you have to get rid of the chemicals, which can be harmful in large amounts. That's where your kidneys come in.

If you reach around and touch the lowest ribs on your back, your hands will be about where your kidneys are. You have two of them, one on each side of your body. They're located deep inside you, protected by a cushion of fat cells, and the armor of your rib cage. They're not very big -- each is about the size of a human fist and weighs about a quarter of a pound. But they do an enormous job. They keep you from poisoning yourself with the waste products your cells create.

You might think of the kidneys as a pair of non-stop washing machines for your blood. Each minute, about a quart of blood passes through the organs and comes out clean. In a lifetime, your kidneys wash your blood over and over and over again -- over 1 million gallons' worth. That's enough to fill a small lake.

Your kidneys may not be very fancy looking or very large, but they have an amazing amount of equipment packed inside them. In fact, these vital organs are one of the most complex parts of your body. Your brain is a more complicated structure -- but the kidneys come in second. Each one contains a network of tiny tubes and bundles of blood vessels that act as filters for your blood. The tiny tubes are called nephrons. Each kidney contains about 1 million of them. If you could stretch the nephrons in your kidneys out and attach them together in a line, it would be about long enough to reach from Washington to Baltimore.

The kidneys' blood-washing business is important. Each day, your kidneys constantly filter your blood. Poisonous substances that would make you very sick or even kill you are washed out of the blood and carried away from the kidneys in a watery fluid called urine. The urine collects in a bag called the bladder. When the bladder gets full, it stretches. This action turns on special nerve cells, which relay a message to your brain. It says, "Time to go to the bathroom," where you get rid of the urine and flush it away. Every day, the human body produces between one and two quarts of urine.

Your kidneys are more than just a human washing machine. They also help keep the chemical balance in your body correct. Regulating the body's chemistry is a very complicated job.

Having two kidneys gives people a kind of built-in insurance policy. In healthy people, each kidney is at work all the time. But sometimes an illness or injury can harm one kidney. Luckily, the other one can continue to work hard enough to keep the person well.

If both kidneys are damaged, medical technology can help. For about 30 years now, doctors have often been able to transplant kidneys from one person into another. Or equipment called a kidney dialysis machine can be used to "wash" a patient's blood when the kidneys have stopped doing their job.

You probably don't give your kidneys much thought. Luckily, you don't need to. They work automatically. Even as you sleep, your own personal clean-up crew is on the job. Tips for Parents

The National Kidney Foundation of the National Capital Area began an education drive in the District's public schools in 1984. A three-lesson curriculum for junior and senior high school students covers kidney structure and function, kidney disease and organ donation. The foundation also provides information to adults -- including a brochure to help parents of children with kidney disease. The brochure, "A Note to Parents of Children with End-Stage Renal Disease," provides practical advice on strategies to help kids take medication, stick to restricted diets and undergo dialysis with the least possible stress. For this publication or other information about kidneys and kidney disease, contact the foundation at 2233 Wisconsin Ave. NW, Suite 320, Washington, D.C. 20007; 337-6600.