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.
Although D-Day has become synonymous with the Allied landing on June 6, 1944, in Normandy, it was used many times before and since. The D in D-Day simply stands for “day,” just as the H in H-Hour stands for “hour.” Both are commonly used codes for the fixed time when a military operation is scheduled to begin. “D minus thirty” means thirty days before a target date while “D plus fifteen” means fifteen days after.
Not all people own dogs and cats. While many Europeans, including the British, French, and Italians, own dogs and cats, other people have different relationships to them. In Islamic tradition, dogs are shunned as unclean and dangerous, and thus it has never been common for Arabs to own pets. However, in Saudi Arabia and Egypt it has become fashionable among the upper class to own dogs and cats. (As early as 3500 B.C.E., Egyptians domesticated wildcats from Africa, which became their treasured pets and honored for their skill in hunting snakes, rats, and mice.) In China, cats are thought to bring good luck and are kept in shops and homes; the country also has about 150 million pet dogs—about one for every nine people.
The Japanese keep birds and crickets as pets. The Inuit Eskimo of northern Canada adopt bear cubs, foxes, birds, and baby seals. And Australian Aborigines capture dingo (wild dog) puppies and raise them for a time before letting them go.
Babies are born with about 300 to 350 bones, but many of these fuse together between birth and maturity to produce an average adult total of 206. Bone counts vary according to the method used to count them, because a structure may be treated as either multiple bones or as a single bone with multiple parts. There are four major types of bones: long bones, short bones, flat bones, and irregular bones. The name of each type of bone reflects the shape of the bone. The shape of the bone also tells about its mechanical function. Bones that do not fall into any of these categories are sesamoid bones and accessory bones.
In the Middle Ages, people treated a dog bite with the ashes of the canine culprit’s hair. The medical logic came from the Romans, who believed that the cure of any ailment, including a hangover, could be found in its cause.
It’s a principle applied in modern medicine with the use of vaccines for immunization. “The hair of the dog” treatment for hangovers advises that to feel better, you should take another drink of the same thing that made you feel so bad.
In the Arctic and Antarctic circles there is at least one day a year when the Sun does not rise and one day when the Sun does not set. This is because of their close location to Earth’s poles.
The Sun does not set on the summer solstice (June 21 in the north and December 21 in the south) and does not rise on the winter solstice (December 21 for the north and June 21 for the south). For this reason, the Arctic and Antarctic are called the “lands of midnight Sun” in the summer and “lands of noon darkness” 12 in the winter.
Kissing bridges are covered bridges with roofs and wooden sides. They are called kissing bridges because people inside the bridge cannot be seen from the outside, making them good places to kiss discreetly. They were first built in the nineteenth century by engineers who designed coverings to protect the structures from the effects of the weather.
More than 10,000 covered bridges were built across the United States between 1805 and the early twentieth century. As of January 1980, only 893 of these covered bridges remained—231 in Pennsylvania alone, where the first one was erected.