Glow-in-the-dark stickers, stars, toys, and clothes, all work by absorbing light and emitting it later. These items contain phosphors, substances such as zinc sulfide that radiate visible light after being energized by natural light. Phosphorescent materials continue to glow after the energizing light is removed. They have electrons that are easily excited to higher energy levels when they absorb light energy.
In phosphorescent materials—such as glow-in-the-dark objects—the excited electrons drop to a lower, but still excited intermediate level and stay there for a period of time before returning to their ground state (original energy level) and emitting the excess energy as visible light.
Early in human history, people used anything that they could find to keep their teeth clean. Usually a thin, sharp object, like a stick, was used to pick out food left between teeth. Chewing on the end of certain sticks would fray the wood, making a kind of brush, which could then be rubbed across the teeth. (Even today, members of primitive tribes chew sticks to keep their teeth clean. The constant chewing produces more saliva than usual, which helps wash food away.) Later, people found that if they rubbed abrasive elements, like salt or chalk, across their teeth, they could get rid of grime. They also used water and pieces of rough cloth to clean their teeth. Toothpicks made of all kinds of materials also became popular. Rich people had jeweled toothpicks made of gold and silver. Toothbrushes for the wealthy, with fancy handles and hog bristles, came into use in the eighteenth century.
Only much later, when cheaper, woodenhandled toothbrushes were made, and the importance of good dental hygiene became known, did most people start to regularly use them.
The suggestion that storks delivered babies came from Scandinavia and was promoted by the writings of Hans Christian Andersen. Storks had a habit of nesting on warm chimneys and would often lift articles from clotheslines then stuff them into these nests, which to children looked like they were stuffing babies down the flue.
The stork is also very nurturing and protective of its young, which helped it become symbolic of good parenthood.
Green plants get nourishment through a chemical process called photosynthesis, Which uses sunlight, carbon dioxide, and water to make simple sugars. Those simple sugars are then changed into starches, proteins, or fats, which give a plant all the energy it needs to perform life processes and to grow. Generally, sunlight (along with carbon dioxide) enters through the surface of a plant’s leaves. The sunlight and carbon dioxide travel to special food-making cells (palisade) deeper in the leaves. Each of these cells contain a green substance called chlorophyll.
Chlorophyll gives plants their green color and traps light energy, allowing food making to take place. Also located in the middle layer of leaves are special cells that make up a plant’s “transportation” systems. Tubelike bundles of cells called xylem tissue carry water and minerals throughout a plant, from its roots to its outermost leaves. Phloem cells, on the other hand, transport the plant’s food supply sugar dissolved in water—from its manufacturing site in leaves to all other cells.
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.