A bicycle is a simple device that increases the power that you have in the muscles of your legs, taking you faster and farther than you could ever run. When you push the pedals of your bike around once, the pedal sprocket—the wheel with teeth to which the pedals are attached—goes around once, too. But it pulls a chain along, one that is connected to a much smaller sprocket (with fewer teeth to grip each link of the chain) in the center of your bike’s rear wheel. This smaller sprocket moves around a number of times for each single turn of your pedals, moving your bike wheels a lot faster than you’re moving your feet! Some bicycles have several “speeds,” which means that they have a number of gears (called derailleurs) that vary the rate at which the wheels turn. These extra sprockets are located at the pedals and rear wheel of a bike, where levers move the driving chain sideways, from one to another.
A special spring system keeps the chain tight when it changes from a larger to a smaller sprocket. Although you might think that a rider would always want the wheels of his or her bike to move as fast as possible for each pedal turn, that is not always the case. When going uphill, for instance, a rider can get more force out of a wheel that turns fewer times, making the task easier.
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.
The word fortnight is a unit of time that equals fourteen days. It comes from the Old English word feorwertyne niht, meaning “fourteen nights.” The term is used in Great Britain, where salaries and most social security benefits are paid on a fortnightly basis, but in the United States people use the term “two weeks.”
In many languages, there is no single word for a two-week period and the equivalent of “fourteen days” has to be used. In Spanish, Italian, French, and Portuguese, the terms quince días, quindicina, quinzaine, and quinzena—all meaning “fifteen days”—are used.
Maybe. Warts are skin infections caused by a common virus called HPV (human papillomavirus). They are perfectly “normal,” in that health researchers estimate that three out of four people will develop a wart some time in their lives, usually on their hands or feet. It can take months or years for a wart to disappear on its own. To speed up the process, some skin doctors recommend wrapping the wart in duct tape until it disappears.
The duct tape removes dead skin from the wart, thereby gradually killing off the wart virus that lives in the skin. It may also trigger the body’s immune system to attack the wart virus. Otherwise the wart can be removed by a doctor with a laser or liquid nitrogen, a substance that freezes the skin, killing the cells.
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.
Yes. Yet unlike a few centuries ago, gold panning today is primarily a recreational activity. Gold nuggets are found in areas where lode deposits and erosion have occurred—for example, in streams, rivers, ravines, and lake areas. All you need is a gold pan, a shovel, and a lot of patience. Both gold mines and gold prospecting sites exist in national parks from near Montgomery, Alabama to Washington, D.C. In addition, North Carolina, South Carolina, Georgia, Virginia, and Alabama have many gold mines and prospecting sites.
These states were America’s main source of gold for 45 years before the California Gold Rush of 1838, when gold was discovered at Sutter’s Mill in Coloma, California. Its news spread like wildfire, resulting in some 300,000 people coming to California to pan for gold. In California, the five counties of Mariposa, Tuolumne, Calaveras, Amador, and El Dorado—nicknamed the “Mother Lode”—still have gold for discovery. In 1837, the U.S. government established gold coin mints in Georgia and North Carolina to avoid transporting the raw gold to the U.S. Mint in Philadelphia, where coins are made.
Pen pals (also called pen friends) are people who regularly write to each other, particularly via postal mail. They are often located in faraway places, such as other states and countries. A pen pal relationship is often used to practice reading and writing in a foreign language, to improve literacy, to learn more about other countries and lifestyles, and to feel connected to people in other parts of the world which helps the world feel like a much smaller place! Pen pals come in all ages, nationalities, and cultures.
Pals may seek new pals based on their own age group or a shared common interest, such as a specific sport or hobby, or they may select someone totally different to gain knowledge about a foreign culture (such as the Far East, Europe, or South America). With the advent of the Internet, a modern version on the traditional pen pal arrangement has developed, and many pen pals also exchange e-mail addresses as well as, or instead of, paper letters. In order to connect with a pen pal, get your mom or dad’s permission first. Often, online sites dedicated to pen pal communication can help you connect with a student your own age.
The deepest hole ever made by humans is in Kola Peninsula in Russia, where in 1989 geologists dug a hole 7.6 miles (12.2 kilometers) deep. The Kola Superdeep Borehole began in the 1970s.
Russian teams used special drilling techniques to dig into the Baltic continental crust, presumed to be about 22 miles (35 kilometers) thick, exposing rocks 2.7 billion years old at the bottom.
Cotton, which comes from flowering Gossypium plants, is a key vegetable fiber used for making clothes, and oil from its seeds can be used in cooking or for making soap. The cotton plant grows in 17 states that make up the U.S. “Cotton Belt”: California, Arizona, New Mexico, Texas, Oklahoma, Arkansas, Missouri, Mississippi, Alabama, Louisiana, Florida, Tennessee, North Carolina, South Carolina, Virginia, Georgia, and Kansas. In the United States, where cotton is no longer picked by hand, machines called pickers or strippers harvest the crops.
Cotton-picking machines have spindles that pick (twist) the seed cotton from the burrs that are attached to plants’ stems. Doffers— a series of circular rubber pads—then remove the seed cotton from the spindles and knock the seed cotton into a conveying system. Conventional cotton stripping machines use rollers equipped with alternating bats and brushes to knock the fluffy white bolls, which contain seeds and hairs, from the plants into a conveyor. After harvest, most of the cotton is pressed into large blocks for storage. These cotton bundles are then transported to the cotton gin, a machine that pulls out the seeds from the cotton bolls.
Wars have taken place since the beginning of recorded history, and they surely occurred before that as well. A war begins when one group of people (the aggressors) tries to force its will on another group of people, and those people fight back. War frequently springs from the differences between people, or from the desire of one group to increase its power or wealth by taking control of another group’s land. Often the aggressors feel that they are superior to the group they want to dominate: they believe that their religion, culture, or even race is better than that of the people they wish to defeat. This sense of superiority makes them feel that it is acceptable to fight to take the land, possessions, and even lives of the “inferior” group, or to force their ways on the dominated people.
Because countries can be very different from one another in government, religion, customs, and ideology (ways of thinking), it is not surprising that nations disagree on many things. But great efforts are usually made to settle the disagreements through discussion and negotiation—a process called diplomacy—before they result in anything as destructive as a war. War usually occurs when diplomacy fails. Because science and technology have allowed us to create such powerful and destructive weapons that can result in such devastating wars, we now have international organizations that work all the time to try to keep peace among nations.