Birds replace their feathers by molting, the periodic shedding of old feathers and the growing of new ones. They do this one to three times each year, although different birds molt at different times of the year. Male goldfinches, for example, molt from a dull greenish yellow to bright yellow during spring. The periodic shedding of feathers and their replacement with new ones makes perfect sense in the animal kingdom.
Feathers are incapable of further growth, and may get worn down, broken, and faded over the year from normal wear-and-tear. Molting replaces these damaged feathers and helps the males look attractive to females, which is why many molts take place during the mating season.
A roller coaster works the same way as a bicycle coasting down a hill. When you ride your bike to the top of a hill, you pedal to get there. Then, to coast down the hill, you take your feet off the pedals and glide down the other side. If the slope is steep enough, you can go very fast. Similarly, a roller coaster is only powered at the beginning of the ride, when the coaster, or train, is pulled up the first hill. When it goes over the top of the hill, the weight of the train itself, pulled downward by gravity, is what keeps the entire unit moving.
There are no cables that pull the train around the track. This conversion of potential energy (stored energy) to kinetic energy (the energy of motion) is What drives the roller coaster, which often reaches 60 miles (96.5 kilometers) per hour. Running wheels guide the train on the track, and friction wheels control the train’s movement to either side of the track. A final set of wheels keeps the train on the track even if it is upside down. Air brakes stop the car as the ride ends.
The Industrial Revolution was an era of sweeping change, as the focus in different societies changed from agricultural to mass-producing and industrial. It began in Great Britain in the 1700s. By the early 1800s it had spread to western Europe and the United States. It was brought about by the introduction of steam-power-driven machinery to manufacturing.
As inventors made new machines that could take over manual labor, sweeping changes in agriculture, textile and metal manufacture, transportation, economic policies, and social structures took place. By the end of the eighteenth century, most finished goods—which had once been made by hand or by simple machines—were produced in quantity by technologically advanced machinery. Factories were built to house the new machines, causing a population shift from rural areas to urban ones.
Although a helicopter doesn’t have wings like an airplane, it uses the same principle of lift to rise and maneuver in the air. The blades of a helicopter’s propeller-like top rotor are shaped just like a plane’s wings—flat on the bottom and rounded on the top—and are likewise adjustable. Instead of rushing forward through the air like a plane does to gather enough lift to fly, a helicopter moves only its (three to six) rotor blades, which are attached to a central shaft driven by an engine.
The rotor blades slice through enough air—creating the changes in surrounding air pressure that produce lift—to achieve flight. Adjusting the angle at which the rotor blades are set helps control a helicopter’s lift and manner of flight. Because the angle of the rotor is adjustable, too, a helicopter has far greater maneuverability than an airplane: besides moving up, down, and forward, it can fly backward and hover in the air.
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.
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.
The head louse is a tiny, wingless insect that lives in human hairs and feeds on very small amounts of blood drawn from the scalp. Although they may sound kind of disgusting, lice appear often on young children in common settings, such as school. Lice aren’t dangerous and they don’t spread disease, but they are contagious and can be very annoying. Their bites may cause the scalp to become itchy and inflamed, and scratching may lead to skin irritation. They can be hard to get rid of, and take lots of work to make them go away and stay away.
Usually, using special medicated shampoos and thoroughly cleaning sheets, carpets, clothing, and personal products, like combs and brushes, gets rid of the pesky insects. Kids should try to avoid head-to-head contact at school (both in classrooms and on the playground) and while playing at home with other children. They also shouldn’t share combs, brushes, hats, scarves, bandanas, ribbons, barrettes, towels, helmets, or other personal care items with anyone else, whether they may have lice or not.
For about six hundred years, from 1250 to 1850, most parts of the world experienced colder and harsher climates than usual. The cooler temperatures were caused by a combination of less solar activity and large volcanic eruptions. Northern Europe’s Little Ice Age took place between 1430 and 1850. When the climate became colder, crops died, and there was widespread famine and disease.
Although it was not a true ice age because it did not get cold enough for long enough to cause ice sheets to grow larger, England experienced some of the coldest winters in its history during the 1820s. Its longest river, the River Thames, froze over regularly and townspeople held Frost Fairs, during which they played games and danced on its icy surface.
The mayor-council is the oldest form of city government in the United States. Its structure is similar to that of the state and national governments, with an elected mayor as chief of the executive branch and an elected council that represents the various neighborhoods, forming the legislative branch. The mayor appoints heads of city departments and other officials.
The mayor also has the power to veto the laws of the city (called ordinances), and prepares the city’s budget. The council passes city laws, sets the tax rate on property, and decides how the city departments spend their money to make the city a better place.
NASA’s space shuttle, also called the Space Transportation System (STS), takes off from Earth like a rocket but lands like an aircraft. It cannot fly to the Moon, but is used to orbit Earth, where the crew can do scientific work, place satellites in orbit, and visit orbiting space stations. Usually five to seven crew members ride the space shuttles, which have all been launched from Kennedy Space Center in Florida. Six shuttles have been built: the first orbiter, Enterprise, was built in 1974 for testing purposes. Five others have gone into space: Columbia, Challenger, Discovery, Atlantis, and Endeavour.
The space shuttle Challenger disintegrated 73 seconds after launch in 1986, and Endeavour was built as a replacement. Columbia broke apart during re-entry in 2003. NASA announced that the space shuttle would no longer be used after 2010, and from 2014 on would be replaced by the Orion, a new space vehicle that is designed to take humans to the Moon and beyond.