What is an avalanche?

An avalanche is a huge mass of ice and snow that breaks away from the side of a mountain and slides downward at great speed. Most avalanches result from weather conditions, such as heavy winds and earth tremors, that cause snow on a mountain slope to become unstable.

A large avalanche in North America might release 300,000 cubic yards of snow—the equivalent of 20 football fields filled 10 feet (3.3 meters) deep with snow. Wintertime, particularly from December to April, is when most avalanches occur.

Where do babies come from?

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.

What causes a rainbow?

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.

How can fish breathe underwater?

Living creatures need oxygen to survive, and fish are no exception. Human beings 68 use their lungs to take in oxygen, and fish breathe using their gills. A fish’s gills are full of blood vessels that absorb the tiny particles of oxygen from the water.

The fish sucks the water in through its mouth and squirts it out through its gills; during this process, the gills take the oxygen from the water into the blood vessels. A fish’s gills are not constructed to take oxygen from the air, so they cannot breathe on dry land.

Do people dream every night?

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.

What was the Challenger disaster?

Seven astronauts were aboard Challenger flight STS-51L when it exploded during liftoff on January 28, 1986. Christa McAuliffe was a Concord, New Hampshire, high school social studies teacher. She and the other six crew members were killed when a solid-fuel booster rocket leak led to a massive fuel tank explosion during liftoff from its launch pad.

NASA’s next flight was Discovery, which was launched on September 28, 1988. After the Challenger disaster, NASA’s three remaining shuttles— Atlantis, Discovery, and Columbia—were rebuilt, each with more than 250 modifications improving safety and performance.

Is it true that at one time there was no oxygen?

Oxygen is necessary for all humans, animals, and plant life to survive. When Earth was first formed, its atmosphere had no oxygen—the colorless, odorless and tasteless gas that makes up about 20 percent of the air we breathe. It had only a deadly combination of hydrogen, methane, ammonia, and hydrogen cyanide.

The hydrogen escaped into space and ultraviolet radiation from the Sun broke down the mixture, leaving only nitrogen and carbon dioxide. Only when life began and photosynthesis (the conversion of light energy into chemical energy by living organisms) occurred did oxygen first appear—about 3.4 billion years ago.

Which are the biggest and smallest bones?

The femur, or thighbone, is the biggest bone in the body. The average femur is 18 inches (45.72 centimeters) long. The longest bone ever recorded was 29.9 inches (75.95 centimeters) long. It was from an 8-foot-tall (2.45 meters) German who died in 1902 in Belgium.

The stirrup (also called the stapes) in the middle ear is the smallest bone in the body. A tiny, U-shaped bone that passes vibrations from the stirrup to the cochlea, it weighs about 0.0004 ounces (0.011 grams) and can measure just one-tenth of an inch.

How big is a hectare?

The hectare (abbreviated ha) is a unit of area equal to 10,000 square meters and used exclusively for measuring land. To get a sense of how big this is, imagine a football field. A football field is almost exactly 100 meters from one end line to the opposite goal line.

Imagine a square of that length on each side, and you’ve got an area of one hectare. There are 100 hectares in one square kilometer, so one square kilometer is the same area as a square that is ten football fields on one side.

What was a “computer” before the electronic age?

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.