The metric system is a decimalized system of measurement. Designed during the French Revolution of the 1790s, the metric system brought order out of the conflicting and confusing traditional systems of weights and measures that were being used in Europe at the time. Prior to the introduction of the metric system, it was common for units of length, land area, and weight to vary, not just from one country to another but from one region to another within the same country.
As the modern nations gradually developed from smaller kingdoms and principalities, confusion multiplied. Length, for example, could be measured in feet, inches, miles, spans, cubits, hands, furlongs, palms, rods, chains, and leagues. Merchants, scientists, and educated people throughout Europe realized that a uniform system was needed, and in 1790 the French National Assembly commissioned the Academy of Science to design a simple decimal-based system of units. The three most common base units in the metric system are the meter, gram, and liter. The meter is a unit of length equal to 3.28 feet; the gram is a unit of mass equal to approximately 0.0022 pounds (about the mass of a paper clip); and the liter is a unit of volume equal to 1.05 quarts. Temperature is expressed in degrees Celsius; 0 degrees Celsius equals 32 degrees Fahrenheit.
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.
Yes and no. Bones are hard connective tissue, made up of bone cells, fat cells, and blood vessels, as well as nonliving materials, including water and minerals. Some bones have a very hard, heavy outer layer made out of compact bone. Under this layer is a lighter layer called spongy bone, which is located inside the end, or head, of a long bone.
Spongy bone is tough and hard, but light, because it has lots of irregularly-shaped sheets and spikes of bone (called trabeculae) that make it porous (full of tiny holes). The soft, jelly-like inner core of bone is called the bone marrow. It is where red blood cells, certain white blood cells, and blood platelets are formed. The jawbone is the hardest bone in your body. Although bones are hard, they are not the hardest substance in the human body: the enamel on your teeth is harder.
America’s founders did debate a bit as to whether or not to force children to attend schools, and they decided to leave such decisions to individual families and local and state governments. The words “education” and “school” do not appear in any of our founding documents, such as the Declaration of Independence, the Constitution, or the Bill of Rights.
Some of our most famous inventors, writers, and politicians were self-taught, learning through mentoring or apprenticeships, conversation, and reading. In 1850, Massachusetts became the first state to institute a compulsory schooling law.
Once seeds are fully developed, they need a good place to grow. If they just fell to the ground beneath their parent plant, they would struggle, competing against each other for sunlight, water, and minerals. Most seeds need to travel—by wind, water, or with the help of insects and other animals—to better places to germinate, or start to grow into new plants. Some seeds, like those from conifer and maple trees, have wings attached. Others, like those of dandelions, have parachutes made of tiny hairs. Both features allow the seeds to be carried great distances by the wind, and they sometimes land in spots that are good for germination. Water carries other seeds to good growing places; the hard, watertight shell of a coconut, for instance, allows it to travel many miles at sea before finding a beach where conditions are suitable for growth.
Seeds sometimes have to wait a long time before they find good places to grow, places where the sun, moisture, and temperature are right. Most seeds are designed for the wait, protected by a hard outer pod (except those of conifers). Some seeds wait years to germinate, and some just never do. But inside each seed pod is a baby plant, or embryo, and endosperm, a supply of starchy food that will be used for early growth if germination takes place. Then a tiny root will reach down into the soil, and a tiny green shoot will reach up, toward the light.
Studies have shown that hundreds, perhaps even thousands, of children and teenagers are accidentally killed by guns in the United States each year. Millions of American kids have access to guns in their homes. People use guns all the time in movies and television shows, and the action scenes in these shows make guns look exciting and powerful. What these shows can’t really convey is the massive, painful destruction an exploding bullet causes when it hits a person’s body. While many kids understand that, in real life, guns can be very dangerous and can cause great harm, most still find guns fascinating. If an adult is supervising and your parents have given their approval, it’s okay to look at and even touch an unloaded gun. But if you are alone or with other kids and you come across a gun, remember that it is not a toy and should not be handled. Guns should never be pointed at another person, even if you intend it as a joke.
If you find a gun in your own house, a friend’s house, or elsewhere, as tempting as it might be to play with it, remember the damage that guns can cause and leave it alone. If you’re away from home, leave right away and tell your parents what happened. Your parents may be upset and worried that you found a gun, but they will be very glad that you told them about it because then they can help you stay safe.
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 governor is responsible for the well-being of his or her state. The details of this job include many hands-on tasks and leadership duties. The governor’s executive powers include the appointment and removal of state officials, the supervision of thousands of executive branch staff, the formulation of the state budget, and the leadership of the state militia as its commander in chief.
Law-making powers include the power to recommend legislation, to call special sessions of the legislature, and to veto measures passed by the legislature. In 43 states, governors have the power to veto (or reject) several parts of a bill without rejecting it altogether. The governor can also pardon (excuse) a criminal or reduce a criminal’s sentence.
Like many inventions, the development of the modern zipper can be traced to a series of events. In 1893, Whitcomb Judson patented and marketed a “clasp locker,” a complicated hook-and-eye shoe fastener. Together with businessman Colonel Lewis Walker, Whitcomb launched the Universal Fastener Company to manufacture the new device. He did not use the word “zipper,” although many people often credit him as the zipper’s creator. Instead, it was Swedish-born Gideon Sundback, an electrical engineer who was hired to work for the Universal Fastener Company, Who gets the credit.
He was responsible for improving Judson’s fastener, and by December 1913, he had designed the modern zipper. Sundback increased the number of fastening elements from four per inch to ten or eleven, had two facing-rows of teeth that pulled into a single piece by a slider, and increased the opening for the teeth guided by the slider. Sundback also created a machine that was able to manufacture the zipper.
Just as the tiny fertilized egg cell from which you began divided again and again to become a baby, the trillions of cells now making up your body continue to divide as you grow. The more cells you have, the bigger you become. Some cells divide to replace worn out cells and others divide to increase the size and change the shape of your body as you mature. Hormones—chemicals that are produced by glands and circulate in your blood—help direct the growth of cells in your body during the process of growing up.
Usually people are fully grown by the time they reach the age of 20. By the time a person is 30, however, the rate at which body cells renew themselves begins to slow down, and signs of aging appear. As time goes on, certain body cells—like those of the brain and nerves—are not replaced when they wear out and die.