Babies grow in their mother’s uterus, a special organ that houses the baby until it is born. At the start of pregnancy, a mother’s egg is fertilized, which makes a new cell. The cell divides quickly into many more cells. At about one week, this tiny mass, called an embryo, sticks to the wall of the uterus, and begins to grow. From the moment of conception, 46 chromosomes and tens of thousands of genes combine to determine a baby’s physical characteristics—the sex, facial features, body type, and color of hair, eyes, and skin. At the eighth week, the embryo is called a fetus.
By the end of the twelfth week, the fetus is completely formed and is able to make a fist, can turn his or her head, and can squint and frown. Until the baby is ready to come out, it grows inside its mother’s uterus. When the baby is ready to be born, at about 40 weeks, the mother starts to feel labor contractions. The uterus squeezes and pushes the baby out of the uterus and into the world.
The practice of using laurels to symbolize victory came from the ancient Greeks. After winning on the battlefield, great warriors were crowned with a wreath of laurels, or bay leaves, to signify their supreme status during a victory parade.
Because the first Olympics consisted largely of war games, the champions were honoured in the same manner: with a laurel, a crown of leaves. To “rest on your laurels” means to quit while you’re ahead.
James Spangler, a janitor at an Ohio department store who suffered from asthma, invented his “electric suction-sweeper,” in 1907 as way of picking up the dust and debris that triggered his health condition. His invention was the first practical domestic vacuum cleaner. It used an electric fan to generate suction, rotating brushes to loosen dirt, a pillowcase for a filter, and a broomstick for a handle.
Because it was heavy and hard to handle, Spangler sold the rights of his invention to his relative, William Hoover, whose redesign of the appliance coincided with the development of the small, high-speed universal motor, in which the same current (either AC or DC) passes through the appliance’s rotor and stator. This gave the vacuum cleaner more horsepower, higher airflow and suction, better engine cooling, and more portability than was possible with the larger, heavier induction motor. Hoover’s model has since been refined, but the mechanics of his vacuum cleaner are still used in vacuum cleaners today.
The funny bone exists in your body, but it’s not a bone at all! The funny bone is a part of the ulnar nerve located at the back of the elbow. If you accidentally bump this area it can cause a tingling sensation toward the front of your forearm.
This tingling or dull pain is caused by the ulnar nerve bumping up against the humerus, the long bone that starts at your elbow and goes up to your shoulder. Although it might feel weird, tapping your funny bone doesn’t do any damage to your elbow, arm, or ulnar nerve.
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.