On Europa, one of Jupiter’s moons, it is possible to ice skate, as long as you wore a space suit. A little smaller than Earth’s moon, Europa is covered in smooth ice. Its gravity is only about one-eighth of planet Earth’s, which makes for great leaps. However, Europa temperatures reach about –328 degrees Fahrenheit (–200 degrees Celsius), which means you would be frozen stiff in a nanosecond.
The only colder object in the solar system is Neptune’s moon, Triton, which has unique “ice volcanoes” and a surface temperature of –391 degrees Fahrenheit (–235 degrees Celsius).
Earrings were used by seamen, especially warriors such as pirates, for very practical reasons and not for decoration. They were given to young sailors as a symbol of their first crossing of the equator, and their purpose was to protect the eardrums during battle.
The pirates, especially those who fired the ships’ cannons during closed combat with the enemy, dangled wads of wax from their earrings to use as earplugs.
Some buildings are called skyscrapers because they are of great height and have an iron or steel frame inside that supports its floors and walls. Before builders figured out how to make such frames, stone or brick walls had to bear the weight of structures, which could not stand up if they were made too high. And tall stone or brick buildings had to have very thick walls on lower floors to bear the weight of the walls and floors above them. These thick walls wasted a lot of useful space. Because cities have limited land, builders experimented with materials and construction methods in an effort to construct taller buildings that were more practical.
Finally, in 1885, William Le Baron Jenny built the first modern skyscraper in Chicago. While just 10 stories high, Which seems short by today’s standards, the Home Insurance Company Building was the first structure to have an internal steel skeleton bear all of its weight. From that point on, tall buildings began to soar into the air, scraping the sky. Just 30 years after the first skyscraper was built, buildings were erected that reached 60 stories high.
A bicycle is a simple device that increases the power that you have in the muscles of your legs, taking you faster and farther than you could ever run. When you push the pedals of your bike around once, the pedal sprocket—the wheel with teeth to which the pedals are attached—goes around once, too. But it pulls a chain along, one that is connected to a much smaller sprocket (with fewer teeth to grip each link of the chain) in the center of your bike’s rear wheel. This smaller sprocket moves around a number of times for each single turn of your pedals, moving your bike wheels a lot faster than you’re moving your feet! Some bicycles have several “speeds,” which means that they have a number of gears (called derailleurs) that vary the rate at which the wheels turn. These extra sprockets are located at the pedals and rear wheel of a bike, where levers move the driving chain sideways, from one to another.
A special spring system keeps the chain tight when it changes from a larger to a smaller sprocket. Although you might think that a rider would always want the wheels of his or her bike to move as fast as possible for each pedal turn, that is not always the case. When going uphill, for instance, a rider can get more force out of a wheel that turns fewer times, making the task easier.
On the outside, most electric cars look exactly like gas-powered cars. An electric car does not have a tailpipe or a gas tank, but the overall structure is basically the same. Instead of a huge engine, an electric car uses an electric motor to convert electric energy stored in batteries into mechanical energy.
Different combinations of generating mechanisms—solar panels, generative braking, internal combustion engines driving a generator, fuel cells— and storage mechanisms are used in electric vehicles.
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.
All living things need water to survive. Without water, the human body stops working properly. Water makes up more than 50 percent of your body weight and a person cannot survive for more than a few days without it. Water flushes toxins out of your organs, carries nutrients to your cells, and provides a moist environment for ear, nose, and throat tissues.
Water is also in lymph, a fluid that is part of your immune system, which helps you fight off illness. You need water to digest your food, to get rid of waste, and to sweat. Too little water in your body leads to dehydration, and it can make you tired and unable to function. Your body gets water from drinking it, but lots of foods, such as fruits and vegetables, contain water too.
Once mail travels close to its destination, it is unloaded at another postal sorting office. A bar code–reading machine scans the letters again, separating them further for delivery to districts, neighborhoods, and streets.
The letters are sent to local post offices, where carriers are given the mail for their delivery areas. Routes may include homes, shops, and office buildings. Carriers who work in farm country, where people live very far apart, may have to travel many miles to deliver the mail each day.
Nobody who has died has been able to come back to tell us about it, so it is impossible to know whether dying hurts. But people who have had “near-death” experiences— those whose hearts have stopped, for instance, but were later restarted— have only good things to report. Most tell of a peaceful sensation of floating above their bodies. A number also describe traveling through a tunnel toward a beautiful light or having loving meetings with friends and relatives who have died before them.
Scientists know that when a person is in a state of very low oxygen—often a condition that precedes death—he or she experiences feelings of euphoria, or great happiness. So as far as we know, the act of dying is not painful at all. Many sick people welcome death. The same wonders of medicine that have allowed people to reach old age have also enabled them to live through long, and sometimes painful, illnesses. Often, death is seen as a welcome end to pain, both for the ill person and for the family and friends who have watched their loved one suffer. People with strong religious faith, too, may fear death less because they believe they will journey to a better place.
Mortimer was Walt Disney’s original name for a cartoon mouse in the historic 1928 cartoon “Plane Crazy.” When Walt came home and told his wife about the little mouse, she didn’t like the name “Mortimer” and suggested that “Mickey” was more pleasant-sounding.
Walt thought about it for a while and then grudgingly gave in, and that’s How Mickey, and not Mortimer, went on to become the foundation of an entertainment empire.