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.
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.
Quickly gobbling up cold ice cream may result in “brain freeze,” also know as an ice cream headache. When the cold object touches the roof of your mouth, the blood vessels contract in order to prevent loss of body heat. As the coldness stops, the blood vessels relax again, quickly increasing blood flow to the brain.
This sudden release is What causes the intense headache sensation. You can relieve brain freeze by quickly warming the roof of your mouth: Touch your tongue to the top of your mouth or, if you can roll your tongue in a ball, press the underside of your tongue (which is warmer) to the roof of your mouth. Slowly sipping room-temperature water or pressing a warm thumb against the roof of the mouth also works for some people.
The coco de mer tree, a palm that only grows today on two islands in the Seychelles, produces both the largest seed (each weighs about 44 pounds [20 kilograms]) and the largest nut in the world. The nut, which takes six to seven years to mature and another two years to germinate, is sometimes called the sea coconut or Seychelles nut. When early explorers first discovered the nut, they thought it came from a mythical tree at the bottom of the sea.
Sixteenth-century European nobles decorated the nut with jewels as collectibles for their private galleries. Today, the coco de mer is a rare protected species.
Cellular, or cell, phones first became available to consumers in the early 1980s, but the technology that made them small and truly portable evolved gradually over the next 10 years or so. By the beginning of the twenty-first century, millions and millions of people in countries all over the world were using cell phones on a daily basis. And it isn’t just adults who enjoy the benefits of completely mobile phone capabilities: in the United States alone, more than 20 percent of teenagers have a cell phone. That translates to at least one in five American teens. The cellular system divides each city into many small cells (a large city can have hundreds). Each cell has its own tower (which contains an antenna as well as transmitters and receivers that send and receive signals). Each tower can handle numerous callers at a given time, and their small size and weaker signal (compared to the radio antennae) means that their signals don’t interfere with those of nearby towers. When you call someone using a cellular phone, your phone is sending and receiving signals via radio waves, invisible bands of energy that work like light rays. In other words, your cell phone is a fancy, high-tech radio. After you dial a friend’s number, your phone must find the closest tower by searching for the strongest signal. Once that signal is located, your phone transmits certain information—like your cell phone number and serial number—that help your service provider make sure you are one of their customers.
Then the mobile telephone switching office (MTSO) finds an available channel where your conversation can take place. The MTSO then completes the connection (all of this happening in a few short seconds) and you are chatting with your friend, without wires or cords to hold you down. If you are sitting in the back seat of the car while talking, and your mom is driving you from one end of town to the other, your call will be switched automatically from one cell tower to the next without any pause in your conversation.
Camels are the only animals with humps. A camel’s hump is a giant mound of fat, Which can weigh as much as 80 pounds (35 kilograms). The hump allows a camel to survive up to two weeks without food. Because camels typically live in the deserts of Africa and the Middle East, where food can be scarce for long stretches, their hump is key to their survival. When camels are born their humps are empty pockets of flexible skin. As a camel grows and begins to form its fatty tissue reserves, the humps begin to fill out and take shape.
The humps also come in handy for humans who have domesticated the camel. For thousands of years, people have used these strong, resilient creatures for transportation and for hauling goods. The two-hump, or Bactrian, camel was domesticated sometime before 2500 B.C.E., probably in northern Iran, northeastern Afghan – istan, and northern Pakistan. The one-hump, or Dromedary, camel was domesticated sometime between 4000 and 2000 B.C.E. in Arabia.
Fixed groups of stars that seem to form a particular shape, such as that of a person, animal, or object, are called constellations. Astronomers have identified 88 constellations and many of them represent characters from Greek and Roman mythology.
For example, the name Hydra, the largest constellation, comes from the water snake monster killed by Hercules in ancient mythology. Some of the constellation names are in Latin; for example, Cygnus means Swan and Scorpius means Scorpion.
Earth Day is a national holiday that was first celebrated on April 22, 1970. It was created by Senator Gaylord Nelson, a Democrat from Wisconsin who was elected to the U.S. Senate in 1962. Senator Nelson decided to set aside one day aside for the entire nation to focus on environmental issues, learn about ways to improve the environment, and protest against the federal government’s unwillingness to help solve problems such as air pollution and the widespread destruction of forests.
After lots of hard work and publicity, on the first Earth Day 20 million Americans gathered at different places from the East to West coasts to hear speeches, participate in community-wide cleanup efforts, and demonstrate to the government that the environment is a major national issue. Ever since then, April 22 has been the date for celebrating Earth Day—a time when the United States (and now many countries all over the world) could participate in educational activities that celebrate Earth and think of new ways to preserve our natural resources. On Earth Day 2008, over 100 million people joined in the effort to celebrate and protect our planet.
People usually read newspapers to get information about current events, things that are happening at the present time or have just occurred. When a good news story breaks, reporters are immediately sent out to gather as much information about the situation as possible and photographers take pictures that add visual information. When they return to the newspaper office, the reporters type their story into a computer, and camera film is developed into photos in a darkroom. The photographs are put into the computer with a device called a scanner. Increasing numbers of photographers use digital cameras, which means their photos do not have to be first developed on paper. They are automatically in digital, or computer-ready, format and can be transmitted over phone lines or via satellites just like e-mail or other electronic files. Once the photos are in digital format, the printed story and the pictures that illustrate it are arranged together. The story may take up part of a newspaper page or may extend for a few pages. Designers arrange all the stories and photos that make up a newspaper into visually appealing, easy-to-read pages on the computer screen. They are then printed out on pieces of clear film. Next, the film print of each newspaper page is laid on a light-sensitive metal plate. When it is exposed to a flash of bright light, shadows of the film’s letters and pictures are left on the plate. The shadows are permanently etched or marked into the plate when it is soaked in acid, which eats some of the metal away. What is left is a perfect copy of the film print of the newspaper page, with its words and pictures appearing as grooves in the metal.
The newspaper page is now ready to be printed on paper. The metal plate is first wrapped around a roller on a motor-driven printing press and coated with ink. After being wiped clean, ink still stays in the grooves. When paper (in big rolls) is passed under the roller, it is pressed into the grooves, and perfectly printed pages appear. This process is repeated for each newspaper page. As you can imagine, printing plants are enormous, with some presses standing three stories tall. These expensive machines (costing tens of millions of dollars) can print and sort up to 70,000 copies of a newspaper per hour. Once the press is done printing and sorting, the newspapers are bundled for delivery the next day to homes and newsstands.
Before Lippershey and Galileo, magnification instruments had not been used to investigate objects beyond Earth. Since their time, far more powerful visiblelight telescopes have been developed along with other types of telescopes capable of “seeing” invisible forms of radiation, such as infrared, ultraviolet, radio, X-ray, and gamma-ray. Today’s optical telescopes (made from glass, lenses, or mirrors) are 100 million times more sensitive than Galileo’s telescope. For example, Hawaii’s twin Keck Tele scopes are the world’s largest optical and infrared telescopes. Each stands eight stories tall and weighs 300 tons.
The Hubble Space Telescope, named after astronomer Edwin Hubble and launched into space in 1990, orbits Earth at a speed of 5 miles (8 kilometers) per second, and beams images back to Earth. Because its position is above the atmosphere (which distorts and blocks the light that reaches Earth) it is able to view the universe more thoroughly and clearly than ground-based telescopes.