Paint coats wood, protecting it from sunlight and rain damage and making it last longer. When early farmers had enough money to paint their barns, they usually used inexpensive paint because the structures were so large. Ferrous oxide, a chemical powder that gives paint its red color, was readily available and cost little. Thrifty farmers in New England, New York, and the upper Midwest region painted their barns red. In those places, red barns remain a tradition. But there are plenty of barns in other parts of the country that are not red. Early farmers that were poor—especially in regions like Appalachia and the South—left their barns unpainted because they did not have the money to do the job.
Unpainted wood usually weathers to a soft gray color. And in places like Pennsylvania, Maryland, and some southern Midwestern states, the most frequently seen barn color is white. Some people think that white barns grew popular when dairy farming became more important after the Civil War; white suggests cleanliness and purity, desirable qualities to be associated with milk production. Special farms where fancy horses or prize livestock were raised sometimes had barns painted unusual colors, like yellow, green, or black.
Air is a mixture of gases that circle Earth, kept in place by gravity. Air makes up Earth’s atmosphere. The air we breathe is 78 percent nitrogen gas, 21 percent oxygen, 0.9 percent argon, and 0.03 percent carbon dioxide, along with water vapor (floating molecules of water).
Also present are traces of other gases and tiny bits of dust, pollen grains from plants, and other solid particles. As our atmosphere extends higher and higher above Earth, toward outer space, air becomes thinner and the combination of gases in the air changes.
On Europa, one of Jupiter’s moons, it is possible to ice skate, as long as you wore a space suit. A little smaller than Earth’s moon, Europa is covered in smooth ice. Its gravity is only about one-eighth of planet Earth’s, which makes for great leaps. However, Europa temperatures reach about –328 degrees Fahrenheit (–200 degrees Celsius), which means you would be frozen stiff in a nanosecond.
The only colder object in the solar system is Neptune’s moon, Triton, which has unique “ice volcanoes” and a surface temperature of –391 degrees Fahrenheit (–235 degrees Celsius).
Earrings were used by seamen, especially warriors such as pirates, for very practical reasons and not for decoration. They were given to young sailors as a symbol of their first crossing of the equator, and their purpose was to protect the eardrums during battle.
The pirates, especially those who fired the ships’ cannons during closed combat with the enemy, dangled wads of wax from their earrings to use as earplugs.
Some buildings are called skyscrapers because they are of great height and have an iron or steel frame inside that supports its floors and walls. Before builders figured out how to make such frames, stone or brick walls had to bear the weight of structures, which could not stand up if they were made too high. And tall stone or brick buildings had to have very thick walls on lower floors to bear the weight of the walls and floors above them. These thick walls wasted a lot of useful space. Because cities have limited land, builders experimented with materials and construction methods in an effort to construct taller buildings that were more practical.
Finally, in 1885, William Le Baron Jenny built the first modern skyscraper in Chicago. While just 10 stories high, Which seems short by today’s standards, the Home Insurance Company Building was the first structure to have an internal steel skeleton bear all of its weight. From that point on, tall buildings began to soar into the air, scraping the sky. Just 30 years after the first skyscraper was built, buildings were erected that reached 60 stories high.
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.
On the outside, most electric cars look exactly like gas-powered cars. An electric car does not have a tailpipe or a gas tank, but the overall structure is basically the same. Instead of a huge engine, an electric car uses an electric motor to convert electric energy stored in batteries into mechanical energy.
Different combinations of generating mechanisms—solar panels, generative braking, internal combustion engines driving a generator, fuel cells— and storage mechanisms are used in electric vehicles.
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.
All living things need water to survive. Without water, the human body stops working properly. Water makes up more than 50 percent of your body weight and a person cannot survive for more than a few days without it. Water flushes toxins out of your organs, carries nutrients to your cells, and provides a moist environment for ear, nose, and throat tissues.
Water is also in lymph, a fluid that is part of your immune system, which helps you fight off illness. You need water to digest your food, to get rid of waste, and to sweat. Too little water in your body leads to dehydration, and it can make you tired and unable to function. Your body gets water from drinking it, but lots of foods, such as fruits and vegetables, contain water too.
Once mail travels close to its destination, it is unloaded at another postal sorting office. A bar code–reading machine scans the letters again, separating them further for delivery to districts, neighborhoods, and streets.
The letters are sent to local post offices, where carriers are given the mail for their delivery areas. Routes may include homes, shops, and office buildings. Carriers who work in farm country, where people live very far apart, may have to travel many miles to deliver the mail each day.