Yes. The average person has three or four dreams each night, with each dream lasting 10 minutes or more. Almost all dreams occur during REM sleep, a period of sleep characterized by fast breathing and heart rates. Scientists do not understand why dreaming is important, but one theory is that the brain is either cataloging the information it acquired during the day and discarding the data it does not want, or is creating scenarios to work through situations that may be causing emotional distress.
Like sleep, most people who are deprived of dreams become disoriented, are unable to concentrate, and may even have hallucinations. Sometimes it is hard to remember our dreams because they are stored in our short-term memory.
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.
Fixed groups of stars that seem to form a particular shape, such as that of a person, animal, or object, are called constellations. Astronomers have identified 88 constellations and many of them represent characters from Greek and Roman mythology.
For example, the name Hydra, the largest constellation, comes from the water snake monster killed by Hercules in ancient mythology. Some of the constellation names are in Latin; for example, Cygnus means Swan and Scorpius means Scorpion.
Our muscles, which make up about half of our body mass, control the way the body moves. Muscles work together all the time, whether we are actively playing sports, or quietly reading and writing. Muscles lie in bands just beneath the surface of the skin. A muscle is made up of thousands of fibers bundled together within a protective sheath that consists of blood vessels and nerves. These nerves can be up to 12 inches (30 centimeters) long. A muscle becomes stronger when you work it, which is why people Who regularly exercise have more defined muscle tone than those Who do not exercise. There are about 660 muscles in the human body. The three types of muscle tissue are skeletal, cardiac, and smooth. The main and most unique characteristic of muscle tissue is its ability to contract, or shorten, making some type of movement possible.
Skeletal muscles hold the bones together, and are often called “voluntary” muscles because the brain controls them. The cardiac muscle, which is found only in the heart, contracts to send blood from the heart into the arteries. The brain sends signals to the cardiac muscle to speed up or slow down its contractions, called the heartbeat. Smooth muscles, located in the internal organs such as the stomach and intestines, help these organs or tissues do their job, such as help you digest and eliminate your food.
Tropical coral reefs are ridgelike or moundlike structures composed of corals and other aquatic organisms. They border the shorelines of more than 100 countries. Although coral reefs comprise less than 0.5 percent of the ocean floor, it is estimated that more than 90 percent of marine species are directly or indirectly dependent on them—they are home to approximately 4,000 species of fish alone.
Reefs protect human populations along coastlines from wave and storm damage by serving as buffers between oceans and near-shore communities. The Great Barrier Reef, located in northeast Australia’s Coral Sea, measures 1,616 miles (2,000 kilometers) in length. It the largest living structure on Earth, and can be seen from the Moon.
A rainbow is an arc that shows all the colors, with their different wavelengths, that make up visible light. Seven colors make up a rainbow, and they always appear in the same order: red, with the longest wavelength, is on the top, followed by orange, yellow, green, blue, indigo (a deep reddish-blue that is often difficult to see), and violet, which has the shortest wavelength. A good way to remember the order of those colors is by taking the first letter of each to spell “ROYGBIV,” pronounced “roy-jee-biv.” A rainbow occurs when sunlight passes through water droplets and is refracted or bent by their rounded shape into separate wavelengths.
Rainbows can sometimes be spotted in the spray of lawn sprinklers, in the mist of waterfalls, and—most spectacularly—in the sky during a rain shower when the Sun is still shining. A rainbow appears in the part of the sky opposite the Sun. Because the Sun must also be low in the sky, near the horizon, late afternoon is the best time to look for a rainbow if the day has been sunny with a few short rain showers or thunderstorms.
The Sun is extremely hot. The surface of the Sun (or its outer visible layer, called the photosphere) is about 10,000 degrees Fahrenheit (5,537 degrees Celsius)— about 50 times the temperature required to boil water.
The core of the Sun, where solar energy is created, reaches 27 million degrees Fahrenheit (15 million degrees Celsius). It is so intense that nuclear reactions take place there.
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.
On a clear, dark night in your backyard, you can see about 2,000 or so stars in the sky, a small fraction of the 100,000 or so stars that make up our galaxy. They seem to twinkle, or change their brightness. In reality, most of the stars are shining with a steady light. The movement of air (sometimes called turbulence) in the atmosphere of Earth causes the starlight to get slightly bent as it travels from the distant star through the atmosphere down to the ground.
This means that some of the light reaches us directly and some gets bent slightly away. To the human eye, this makes the star seem to twinkle.
Jupiter, Saturn, Uranus, and Neptune have rings—or thin belts of rocks—around them. Jupiter’s ring is thin and dark, and cannot be seen from Earth. Saturn’s rings are bright, wide, and colorful. Uranus has nine dark rings around it, and Neptune’s rings are also dark, but contain a few bright arcs.
At one time all of the planets, Earth included, had rings. These rings were unstable and the material was either lost in space or collected into the satellites of these planets.