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 word computer first appeared in the seventeenth century as the job title of a person who did calculations as an occupation.
Although slide rules were sometimes called computers, it wasn’t until the 1940s, with the development of massive electronic data machines, that the human occupation of computing became obsolete. These mechanical devices became known as computers.
In 1943 and 1944, the British government developed two Colossus computers. These huge machines were electronic computing devices used by British code breakers to read encrypted German messages during World War II. Dubbed “Colossus Mark 1” and “Colossus Mark 2” these devises were the world’s first programmable, digital, electronic, computing machines.
Based on concepts of the British mathematician Alan M. Turing, the mathematician Max Newman and engineer Tommy Flowers designed and built the machines, which used vacuum tubes (thermionic valves) to perform the calculations. The Colossus hardware and blueprints were destroyed as part of an effort to keep the project secret. However, based on notes in engineers’ logs and other information, in 2007 a functional replica of a Colossus computer was completed. The computer is on display at the Bletchley Park Museum in Milton Keynes, Buckinghamshire, England.
Once in a while the Moon passes directly in front of the Sun as it makes its way around Earth. It temporarily blocks out the Sun, casting a shadow on a portion of Earth that is experiencing day.
When this total eclipse of the Sun—a solar eclipse occurs, the part of Earth affected becomes dark and cold until the Moon passes by. Surrounding areas experience a partial eclipse, when just part of the Sun is temporarily covered by the Moon.
First built in 1960 by American physicist Theodore Maiman, lasers are machines that produce intense beams of high-energy light. Laser light is more powerful than ordinary light because all its rays have the same wavelength and move together in exactly the same direction, allowing them to be focused in a narrow beam with great precision.
Laser light beams vary in strength, depending on the materials and amount of energy used to make them. Lasers can melt, burn, or cut through a variety of different surfaces, from hard metal to the delicate human body, which is why they are often used in surgery today. Lasers can be used to make precise measurements, to reshape corneas to correct poor vision, to transmit telephone signals, to guide weapons, and to read supermarket bar codes.
Our muscles, which make up about half of our body mass, control the way the body moves. Muscles work together all the time, whether we are actively playing sports, or quietly reading and writing. Muscles lie in bands just beneath the surface of the skin. A muscle is made up of thousands of fibers bundled together within a protective sheath that consists of blood vessels and nerves. These nerves can be up to 12 inches (30 centimeters) long. A muscle becomes stronger when you work it, which is why people Who regularly exercise have more defined muscle tone than those Who do not exercise. There are about 660 muscles in the human body. The three types of muscle tissue are skeletal, cardiac, and smooth. The main and most unique characteristic of muscle tissue is its ability to contract, or shorten, making some type of movement possible.
Skeletal muscles hold the bones together, and are often called “voluntary” muscles because the brain controls them. The cardiac muscle, which is found only in the heart, contracts to send blood from the heart into the arteries. The brain sends signals to the cardiac muscle to speed up or slow down its contractions, called the heartbeat. Smooth muscles, located in the internal organs such as the stomach and intestines, help these organs or tissues do their job, such as help you digest and eliminate your food.
Scientists often compare Earth to an onion because the planet is made up of many layers of rocks of different densities. On the outside, there is a think crust of hard, cold rock, which is about 4 miles (7 kilometers) thick under the oceans and 22 miles (35 kilometers) thick under the continents. The crust—the layer we live on—surrounds a hard, rocky surface that marks the top of the mantle, called the lithosphere. Most of Earth is made of its mantle, Which goes almost halfway down to Earth’s center.
At the very center is Earth’s core, which has a center of solid iron and nickel about the size of the Moon (called the inner core) and a molten exterior (called the outer core). The temperature of Earth increases about 36 degrees Fahrenheit (2.2 degrees Celsius) for every 0.62 miles (1 kilometer) down you go, reaching temperatures as high as 11,000 degrees Fahrenheit (6,093 degrees Celsius) at its center.
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.
Most plants have leaves, even if they do not look like leaves. For example, blades of grass are really leaves. Mushrooms and other fungi do not have leaves, and seaweeds and lichens do not have leaves. Seaweed, a type of algae, also does not have flowers or roots. As an underwater plant, it usually clings to stones, shells, and rocks with its holdfast, a part of the plant that looks like roots. Unlike other plants that feed through their roots, seaweed takes its nutrients from the water in which it grows.
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.