The word “planet” comes from the Greek word for “wanderer.” Ancient astronomers defined planets as objects that moved in the night sky against a background of fixed stars. Today, astronomers define a planet as an object in orbit around the Sun that is large enough (massive enough) to have its self-gravity pull itself into a round, or nearly round, shape.
In addition, a planet orbits in a clear path around the Sun—there are no other bodies in its path that it must “sweep up” (or clear) as it moves around the Sun.
The Statue of Liberty stands for many of the nation’s most cherished ideals: freedom, equality, and democracy. Perhaps most importantly to the millions of immigrants for whom the statue was one of their first sights of the United States, it stands for the ideal of opportunity—the chance to begin a new life, in a new land. While their lives in the United States were frequently difficult, for millions of immigrants America offered the chance to escape from grinding poverty and abusive governments in other lands. Standing in the midst of New York Harbor, the point of entry into the United States for so many immigrants arriving on ships from other countries, the Statue of Liberty has been a powerful symbol of opportunity for more than 100 years.
A poem called “The New Colossus,” written by Emma Lazarus, was mounted on the statue’s pedestal in the early 1900s. Its famous lines include these words that Lazarus imagined Lady Liberty to be saying: Give me your tired, your poor, Your huddled masses yearning to breathe free, The wretched refuse of your teeming shore; Send these, the homeless, tempest-tost to me, I lift my lamp beside the golden door!
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.
A cactus-like plant of the lily family, aloe vera grows wild in Madagascar and on the African continent. It also is cultivated in Japan, the Caribbean, the Mediterranean, and the United States. People around the world have used its gooey, jelly-like sap for healing and cosmetic purposes.
Aloe extracts can be used to treat digestive problems, including constipation, and aloe oils are used in cosmetic creams to help keep skin soft and to treat minor skin irritations.
Dams, which are structures that hold back water, have been built since ancient times. They are usually made of earth, rock, brick, or concrete—or a combination of these things. They are constructed to control the flow of water in a river, and they are built for a number of reasons. One reason is to prevent flooding. Heavy rains in high country may cause water levels in a river to rise. As the river flows downhill, it may overflow its banks, flooding communities located downstream. A dam can prevent this by stopping or slowing rushing water, allowing it to be released at a controlled rate. Dams are also frequently used to store water for general use and farming. When a river’s flow is restricted by a dam, water often spreads out behind the dam to form a lake or reservoir in the river valley. That water can then be used as needed, preventing water shortages and crop damage during long periods of dry weather.
A great number of dams today are used to make electricity. Such hydroelectric dams are built very tall, to create a great difference in the height of the water level behind and in front of it. High water behind a dam passes through gates in the dam wall that allow it to fall to the river far below. As the water falls, it flows past huge blades called turbines; the turbines run generators that make electricity. One of the world’s largest and most productive hydroelectric dams is the Hoover Dam, located on the Colorado River between Nevada and Arizona. Built in the 1930s, it is 726 feet (221 meters) high and 1,244 feet (379 meters) long. Its reservoir (Lake Mead)—the world’s largest— supplies water to several states, allowing huge regions of naturally dry terrain in southern California, Arizona, and Mexico to flourish. Many modern dams are used for all three purposes: flood control, water storage, and hydroelectric power.
Antibiotics are medicines that help the human body fight bacteria, either by directly killing the offending germs or by weakening them so that the body’s own immune system can fight and kill them more easily.
The most widely known antibiotic is penicillin, which is made from mold. Penicillin kills bacteria by interfering with the formation of the cell walls or cell contents of the bacteria.
Most adult insects, including bees and dragonflies, have two large compound eyes, made up of separate, sometimes thousands, of lenses. They all point in different directions to give the insect a very wide field of vision.
The lenses also help the insect see movement, enabling it to react quickly to seize its prey or escape danger. You can witness this yourself as you try to swat a fly in your home it’s almost impossible to catch a flying insect!
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.
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.
The word “toadstool” dates to the Middle Ages when it was associated with the toad, Which was thought to be poisonous. Toadstools, with their stool-like shape, are really mushrooms that are poisonous or inedible.
For example, the bright red “fly agaric” toadstool’s juice was once used to make a remedy for killing bugs. It is a very poisonous mushroom that still lives today and should be avoided.