Airplanes function according to a complex mix of aerodynamic principles—theories that explain the motion of air and the actions of bodies moving through that air. Airplanes get their power from engines. Small planes generally use piston engines, Which turn propellers that push aircraft through the air in the same way that boat propellers push vessels through water. But bigger planes use jet engines, powered by burning fuel. These engines expel great amounts of air that thrust a plane forward and up. An airplane must be in constant motion—its wings slicing through rushing air to create lift—in order to stay up; moving air is also required to steer it. In order to get enough lift to rise into the air on takeoff, an airplane has to travel along the ground first at great speed.
Airplanes are able to lift into the air and stay there because of the shape of their wings. An airplane wing is flat on the bottom and curved on the top. When a plane’s engines push it forward, air divides to travel around its wings. The air that passes over the larger curved top moves faster than the air that passes under the flat bottom. The faster-moving air on top becomes thinner and has lower pressure than the air below, which pushes the wing up. Uneven air pressure caused by the shape of an airplane’s wings, then, creates a force called lift, which allows an aircraft to fly.
Earth is more active, in terms of both geology and weather, which makes it hard for craters to remain. Even those craters scientists can see on the surface—which may be millions of years old—have been overgrown by vegetation, weathered by wind and rain, and changed by earthquakes and landslides. The Moon, meanwhile, is geologically quiet and has almost no weather, so its hundreds of thousands of craters are easy to see.
The craters are the result of both meteorites and volcanic activity. Interestingly, some of the oldest Earth rocks might be awaiting discovery on the Moon, having been blasted there billions of years ago by asteroid impacts that shook both worlds.
Carved into the southeast face of a mountain in South Dakota are the faces of four presidents: George Washington, Thomas Jefferson, Theodore Roosevelt, and Abraham Lincoln. Known as Mount Rushmore, these 68-feet (20.7 meters) high granite sculptures were the brainchild of South Dakota state historian Doane Robinson. In 1923, he conceived the project to attract more people to the Black Hills of South Dakota.
Congress passed legislation that authorized the carving in what is today known as Black Hills National Forest. In 1927, the sculptor Gutzon Borglum and 400 workers began the project, using dynamite to remove unwanted rock. Mount Rushmore was completed in 1941.
The human body is a complicated living machine in which various systems work together as a functioning whole. All the parts of the body—including hundreds of rock-hard bones and quarts of blood—are made up of cells, about 100 trillion (100,000,000,000,000) cells in all! Twenty-two internal organs—the large body parts like the heart, lungs, liver, and kidneys—perform special jobs and work together to form the different body systems. There are eight key systems in the body. The muscular system, made up of more than 600 muscles, enables our bodies to make all of their movements. The circulatory system carries oxygen-rich blood throughout the body. The skeletal system is made of the bones that form the skeleton and give the body its shape. People breathe using the respiratory system. The body’s heat-control system is called the integumentary system, which is made up of skin, hair, nails, and sweat glands. The reproductive system creates new life.
The nervous system processes information from both inside and outside the body and sends messages, via its nerves, to different parts of the body. And the digestive system helps us digest our food and nutrients and gives us energy to go through the day. There are other systems, too, that help the body sustain life, including the immune system, which fights off invading viruses and diseases, and the urinary system, which helps keep the inside of the body clean and eliminates waste. The endocrine system, made up of glands, sends hormones around the body to trigger growth and to control other activities. These body systems work together to keep all human beings alive and healthy.
Without the Sun, life on Earth would not exist. The planet would be a frozen dark ball, drifting in space. The Sun provides light, heat, and energy, which stirs up the atmosphere to create winds and rain. With it, plants grow, and animals and humans eat.
However, the Sun’s heat output changes over time, which affects our daily lives, the climate, and our satellite communications.
The Japanese word tsunami (pronounced soo-NAH-mee) means “harbor wave.” It is a huge volume of moving seawater—kind of like a giant wave—that can travel for thousands of miles across the sea and then approach the shoreline with the strength to destroy buildings, trees, wildlife, and people. Tsunamis can be triggered by an undersea earthquake, landslide, or volcanic eruption. The most frequent tsunami-maker is an undersea earthquake, which buckles the seafloor and displaces large volumes of seawater, creating a tsunami. This unique ocean event is not related to tides, although it is sometimes mistakenly called a tidal wave.
A virus is a malicious software program that infects computer files or hard disk drives and then makes copies of itself. Many activities that kids do online can leave computers vulnerable to viruses. E-mail attachments are a common means of distributing viruses, but viruses can also be downloaded when you share files and open instant message attachments. In order to keep your computer safe, never open an e-mail attachment you haven’t requested.
Send an e-mail to friends to confirm that they meant to send you an attachment. Also, you can configure your instant messaging program so you can’t receive files from other users. Never download any program without checking with a parent first. You can protect your computer by always running up-to-date firewall software, by running antivirus software regularly, and by periodically scanning your computer for spyware or other unwanted software and immediately removing it.
Libraries offer books for people of all ages, and much, much more—they are places of learning and discovery for everyone. Besides books, public libraries offer videos, DVDs, free access to computers and the Internet, and many literacy-related programs.
For elementary school children, there are variations of the read-alouds and storytelling hours that often include discussions and presentations by the children themselves, as well as summer reading programs. For middle-school kids, there may also be book talks, summer reading programs, creative writing seminars, drama groups, and poetry readings. The more you read, the more you learn! In addition, the library is a place to find information and help with schoolwork. Your school library may offer some of these services as well.
The brain is the body’s command center; everything we do—eating, talking, walking, thinking, remembering, sleeping—is controlled and processed by the brain. As the most complex organ in the human body, the brain tells us what’s going on outside our bodies (whether we are cold or hot, for instance, or whether the person we see coming toward us is a friend or a stranger) as well as what’s going on inside our bodies (whether we have an infection or a broken bone, or whether we feel happy or sad).
The key to the body’s nervous system, the brain contains between 10 billion and 100 billion nerve cells, or neurons. Neurons combine to form the body’s nerves, thin cords that spread from head to toe and all parts in between. Neurons take in and send out electrical signals, called impulses, that control or respond to everything your body does and feels. The brain is like a very busy, high-speed post office, constantly receiving messages and sending them out all the time; it handles millions of nerve impulses every second.
Today there are some 4,300 religions in the world. Nearly 75 percent of the world’s population practices one of the five most influential religions of the world: Christianity, Buddhism, Hinduism, Islam, and Judaism. Christianity, which is based on the teachings of Jesus Christ, who preached in Palestine about 2,000 years ago, is the most widely practiced religion in the world today, with 2.1 billion followers.
The second most practiced religion is Islam, with 1.3 billion followers.