Dams, which are structures that hold back water, have been built since ancient times. They are usually made of earth, rock, brick, or concrete—or a combination of these things. They are constructed to control the flow of water in a river, and they are built for a number of reasons. One reason is to prevent flooding. Heavy rains in high country may cause water levels in a river to rise. As the river flows downhill, it may overflow its banks, flooding communities located downstream. A dam can prevent this by stopping or slowing rushing water, allowing it to be released at a controlled rate. Dams are also frequently used to store water for general use and farming. When a river’s flow is restricted by a dam, water often spreads out behind the dam to form a lake or reservoir in the river valley. That water can then be used as needed, preventing water shortages and crop damage during long periods of dry weather.
A great number of dams today are used to make electricity. Such hydroelectric dams are built very tall, to create a great difference in the height of the water level behind and in front of it. High water behind a dam passes through gates in the dam wall that allow it to fall to the river far below. As the water falls, it flows past huge blades called turbines; the turbines run generators that make electricity. One of the world’s largest and most productive hydroelectric dams is the Hoover Dam, located on the Colorado River between Nevada and Arizona. Built in the 1930s, it is 726 feet (221 meters) high and 1,244 feet (379 meters) long. Its reservoir (Lake Mead)—the world’s largest— supplies water to several states, allowing huge regions of naturally dry terrain in southern California, Arizona, and Mexico to flourish. Many modern dams are used for all three purposes: flood control, water storage, and hydroelectric power.
Yes. Washing your hands with soap and water cleans them of pathogens (bacteria and viruses) and chemicals that can cause disease. Hot water is not enough to clean your hands. Using soap adds to the time spent washing and breaks down the grease and dirt that carry most germs.
The most important times to wash your hands with soap and water are after you use the toilet or before handling food. When not washed with soap, hands that have been in contact with human or animal feces, bodily fluids like mucus, and contaminated foods or water can transport bacteria, viruses, and parasites to others. When done thoroughly and at least for 20 seconds, hand washing can prevent all types of illness and disease, skin infections, and eye infections.
“Dog days” are the hot, humid days of summer that usually take place in the Northern Hemisphere in July and August typically between July 3 and August 11. The days get their name from the dog star Sirius of the constellation Canis Major.
At this time of year, Sirius, the brightest visible star, rises in the east at the same time as the Sun in the northern hemisphere. Ancient Egyptians believed that the heat of this brilliant star added to the Sun’s heat to create this hot weather and they blamed the star for everything from withering droughts to sickness.
In the United States you are considered a grown-up when you reach the age of 18. You are no longer legally connected with your parents, and you are entitled to the rights—and expected to fulfill the duties—of an adult American citizen. (You may vote and be called for military service, for instance.)
There is a good chance, though, that when you are 18 your body has not yet reached full maturity. Many people continue to grow for a few more years. Most are fully grown—at least in height—by the time they are 20 years old, though boys may keep on growing until they are 23.
Water is vital to the survival of everything on the planet and is limited in supply. Earth might seem like it has abundant water, but in fact only 1 percent is available for human use. While the population and the demand on freshwater resources are increasing (each person uses about 12,000 gallons of water every year), supply remains the same. Water is constantly being cleaned and recycled through Earth’s water cycle, yet we still need to conserve it because people use up Earth’s freshwater faster than it can naturally be replenished.
When you use water wisely, you help the environment. You save water for fish and animals, help preserve drinking water supplies, and ease the job of wastewater treatment plants—the less water you send down the drain, the less work these plants have to do to make water clean again. When you use water wisely, you also save the energy that your water supplier uses to treat and move water to you, and the energy your family uses to heat your water. Your family pays for the water you use, so if you use less water, you’ll have more money left to spend on other things.
Scientists do not know exactly why people need sleep, but studies show that sleep is necessary for survival. Sleep appears to be necessary for the nervous system to work properly. While too little sleep one night may leave us feeling drowsy and unable to concentrate the next day, a long period of too little sleep leads to poor memory and physical performance.
Hallucinations (seeing things that aren’t really there), vision problems, and mood swings may develop if sleep deprivation continues.
Although everyone eats different foods, doctors and nutritionists (people who plan food and nutrition programs) generally agree that a healthy diet is one loaded with fruits, vegetables, whole grains, and fat-free or low-fat milk and milk products; one that includes lean meats, poultry, fish, beans, eggs, and nuts; and one that is low in saturated fats (like those fats found in butter), trans fats (also called partially hydrogenated oils, found in foods like French fries and donuts), cholesterol, salt (sodium), and added sugars.
Foods like processed cheese that squirts from a can, frosted pastries filled with sugary jam, and deep-fried fish, nuggets, or French fries do not provide the types of nutrients your body needs to grow, and are sometimes called “empty” foods. For example, 1 ounce of potato chips has 152 calories and 10 grams of fat (3 grams of which are saturated fat).
The head louse is a tiny, wingless insect that lives in human hairs and feeds on very small amounts of blood drawn from the scalp. Although they may sound kind of disgusting, lice appear often on young children in common settings, such as school. Lice aren’t dangerous and they don’t spread disease, but they are contagious and can be very annoying. Their bites may cause the scalp to become itchy and inflamed, and scratching may lead to skin irritation. They can be hard to get rid of, and take lots of work to make them go away and stay away.
Usually, using special medicated shampoos and thoroughly cleaning sheets, carpets, clothing, and personal products, like combs and brushes, gets rid of the pesky insects. Kids should try to avoid head-to-head contact at school (both in classrooms and on the playground) and while playing at home with other children. They also shouldn’t share combs, brushes, hats, scarves, bandanas, ribbons, barrettes, towels, helmets, or other personal care items with anyone else, whether they may have lice or not.
Technically, no. While it is responsible for receiving and transmitting all messages of pain for the whole body, the brain itself does not have pain receptors.
That means that, if you could somehow gain access to another person’s brain, you could poke it or pinch it and that person would not feel the pain.
Itchy, red skin may be the sign of athlete’s foot, a skin infection caused by a moldlike fungus. The fungus needs a warm, moist environment to live and often grows on the floors of locker rooms and public showers and in swimming pools and whirlpools. It also loves stinky old tennis shoes. When a foot comes in contact with the fungus, it becomes red and itchy. Sometimes, moist, white, scaly lesions or sores develop between the toes and spread to the soles of the feet.
In boys, sometimes athlete’s foot fungus spreads to the groin area, where it is called “jock itch.” The fungus sometimes spreads from one location to another as it is picked up on a bath towel, and the groin area, which is warm and moist, helps the fungus flourish.