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.
First built in 1960 by American physicist Theodore Maiman, lasers are machines that produce intense beams of high-energy light. Laser light is more powerful than ordinary light because all its rays have the same wavelength and move together in exactly the same direction, allowing them to be focused in a narrow beam with great precision.
Laser light beams vary in strength, depending on the materials and amount of energy used to make them. Lasers can melt, burn, or cut through a variety of different surfaces, from hard metal to the delicate human body, which is why they are often used in surgery today. Lasers can be used to make precise measurements, to reshape corneas to correct poor vision, to transmit telephone signals, to guide weapons, and to read supermarket bar codes.
Rain clouds are generally dark gray because light cannot penetrate them due to the deep and densely packed water droplets and ice inside the cloud. Generally, the color of a cloud depends on the cloud’s relationship to the sunlight: Clouds appear gray when they block sunlight.
The thicker the cloud, the more light it blocks. When a cloud is about 3,000 feet (914 meters) thick, hardly any sunlight will make its way through the cloud.
Pythagoras was one of the first Greek mathematical thinkers. He is known for proving and teaching the Pythagorean Theorem, which says that in a right triangle, the sum of the squares of the two right-angle sides will always be the same as the square of the hypotenuse (the long side). He lived in the 500s B.C.E., and spent most of his life in the Greek colonies in Sicily and southern Italy. He had a group of followers Who studied with him and taught other people what he had taught them.
The Pythagoreans were known for their pure lives (they did not eat beans, for example, because they believed that beans were not a pure food). They wore their hair long, wore only simple clothing, and went barefoot. Both men and women Pythagoreans were interested in philosophy, but especially in music and mathematics, which they believed were two ways of making order out of chaos. Nichomachus of Gerasa was a Pythagorean of the first and second centuries. His ideas about arithmetic built upon Pythagoras’s ideas of the harmonic sounds of the motions of the planets and the proportional relationships of numbers.
The Bill of Rights limits the ability of the government to intrude upon certain individual liberties, guaranteeing freedom of speech, press, assembly, and religion to all people. Nearly two-thirds of the Bill of Rights was written to safeguard the rights of those suspected or accused of a crime, providing for due process of law, fair trials, freedom from self-incrimination and from cruel and unusual punishment, and protection against being tried twice in court for the same crime.
Since the adoption of the Bill of Rights, only 17 additional amendments have been added to the Constitution. While a number of these amendments revised how the federal government is structured and operates, many expanded individual rights and freedoms.
The Internet is an amazing place where you can find information on all kinds of things. You can chat with friends, e-mail long-distance pen pals, and read what other people are saying about things you are interested in. But just as you should not talk to strangers when you are in the outside world, you should also use caution when chatting in the cyber world. Unfortunately, there are people surfing the Web who present a threat to kids.
They may be adults posing as another kid or somehow lying to you about who they are and what they want. To be safe, never give anyone you don’t know personal information about yourself online—including your name, address, phone number, or e-mail password. And never agree to meet a person you’ve chatted with online, even if that person seems friendly or harmless. Let your parents know if a stranger is sending you e-mail or instant messages.
The Earth is not round, but slightly squashed; it’s diameter at the equator (the imaginary line on Earth’s surface that divides Earth into a Northern and a Southern Hemisphere) is about 24 miles (38 kilometers) greater than its diameter at the poles. Why? Because the planet is constantly spinning, forcing material out toward the equator. The Earth’s surface is both smooth and bumpy, with vast oceans, tall mountains, rolling plains, canyons, swamps, and deserts.
The tallest mountain on our planet, Mount Everest in the Himalayas, stands at an altitude of 29,108 feet (8,872 meters), while Africa’s Sahara, the largest desert on Earth, spans over 2.1 million square miles (500,000 square kilometers) of land.
Scientists have conflicting opinions about whether dinosaurs were warm-blooded or cold-blooded. Some paleontologists think that all dinosaurs were “warm-blooded” in the same way that modern birds and mammals are, with a high rate of metabolism (body chemistry). Some scientists think they were “cold-blooded,” much like modern reptiles. Some scientists think that very big dinosaurs could have had warm bodies because of their large body size, just as some sea turtles do today. It may be that some dinosaurs were warm-blooded—the problem is that it is hard to find evidence that shows with certainty what dinosaur metabolisms were like. An understanding of dinosaur metabolism helps paleontologists understand the behavior of dinosaurs. If they were cold-blooded, they were most probably sluggish, with only occasional bursts of quickness.
In addition, they probably would not have been very smart creatures. Like modern crocodiles, they probably spent most of their time basking in the sun, moving only to get more food. On the other hand, if dinosaurs were warm-blooded, then they were probably active, social animals. They would have been quick, alert, and intelligent. They would have spent much of their time actively grazing, like the modern antelope, or hunting in packs, like the lion.
The good thing about fission-generated nuclear energy is that very little fuel is needed to produce huge amounts of energy. (Two pounds of nuclear fuel could produce as much energy as 6.5 million pounds of coal, for instance!) The challenging part is that the process must be very carefully controlled. (In a nuclear reactor, control rods that absorb neutrons are moved in and out of the core to control the process.) If it isn’t controlled, the result could be a build up of pressure within the reactor. If this continues, radioactive gases might be released along with steam. It was a situation like this that happened at the Chernobyl plant in the Soviet Union in 1986, resulting in radioactive pollution that still exists today. An uncontrolled nuclear reaction can cause harmful radioactive materials (such as iodine isotopes that can cause thyroid cancer) to be released into the environment. This by-product of nuclear fission is a problem connected with nuclear power. Nuclear reactors are encased in thick layers of steel and concrete to keep radiation from escaping.
And because leftover nuclear fuel is highly radioactive, it must be carefully stored far away from people for decades or even centuries before it is safe again. Transporting and disposing of dangerous waste is another challenge presented by nuclear power; at present, used fuel is sealed in safety containers and buried deep underground. The nuclear process that we get our power from is called fission, where atomic nuclei that break apart produce great energy and heat. But nuclear power can also be created by a process called fusion, where atomic nuclei join together. Scientists are still working on creating a satisfactory fusion reactor. The Sun produces its great energy and heat through the nuclear fusion of its hydrogen gases.
The brain is the body’s command center; everything we do—eating, talking, walking, thinking, remembering, sleeping—is controlled and processed by the brain. As the most complex organ in the human body, the brain tells us what’s going on outside our bodies (whether we are cold or hot, for instance, or whether the person we see coming toward us is a friend or a stranger) as well as what’s going on inside our bodies (whether we have an infection or a broken bone, or whether we feel happy or sad).
The key to the body’s nervous system, the brain contains between 10 billion and 100 billion nerve cells, or neurons. Neurons combine to form the body’s nerves, thin cords that spread from head to toe and all parts in between. Neurons take in and send out electrical signals, called impulses, that control or respond to everything your body does and feels. The brain is like a very busy, high-speed post office, constantly receiving messages and sending them out all the time; it handles millions of nerve impulses every second.