Central Idea

AC vs. DC

In the 1880s, the War of the Currents began. Thomas Edison developed direct current, or DC, a type of electricity that flows in one direction. By 1887, DC was in use around the United States. It had some drawbacks, though. Converting it to lower or higher voltages was difficult; that is, the current's force couldn't be changed easily. Transmitted at the low voltage needed for use in homes and businesses, it lost power after traveling only a mile. Meanwhile, Nikola Tesla was working with alternating current, or AC. AC reverses direction many times per second and can be changed to different voltages. At high voltage, it can be transmitted efficiently, traveling hundreds of miles with little energy loss. Because of its advantages, AC triumphed by the end of the nineteenth century.

What is the central idea of the passage?

• AC reverses direction and can be converted to lower to higher voltage.

• AC has distinct advantages over DC, so AC won the War of the Currents.

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African Wild Dogs: Ultimate Predators?

Many books, papers, and television shows have created an image of the African wild dog as the ultimate predator. According to these sources, the dogs' coordinated teamwork and their unusual endurance help them catch about eighty percent of their prey. A 2012 study tested that notion. Researchers fit a pack of wild dogs with special collars that recorded the dogs' positions and speeds. The data that the collars recorded allowed the researchers to learn what the dogs did when they weren't being observed. The results provided a surprising contrast with the dogs' fierce reputation. The dogs never used teamwork in their hunts; they chased their prey over short distances as opposed to long runs; and instead of eighty percent, they brought down only about sixteen percent of their prey.

What is the central idea of the passage?

• African wild dogs have a reputation as fierce hunters who work in teams, but new data suggests otherwise.

• A study shows that African wild dogs catch about sixteen percent of their prey and do not use teamwork in hunting.

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Naming Pluto

When "Planet X" was discovered in 1930, astronomers began debating what to call it. An eleven-year-old girl named Venetia Burney suggested the name "Pluto" to her grandfather, who passed the idea along to a friend who was a professor of astronomy. The professor, in turn, immediately forwarded the suggestion to scientists at the Lowell Observatory, where the planet had been discovered. The astronomers appreciated the fact that Pluto was the ancient Romans' name for the god of the underworld, making it a fitting moniker for such a cold and distant celestial body. In addition, its first two letters matched the initials of Percival Lowell, the astronomer who first proposed the existence of Planet X. The matter was eventually put to a vote, with Pluto the hands-down winner.

What is the central idea of the passage?

• Pluto is a good name for a cold, distant planet because it is the name of the Roman god of the underworld.

• After an eleven-year-old girl suggest “Pluto,” astronomers chose it as the name of a newly discovered planet.

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The Evolving Human Skeleton

Our early ancestors were hunter-gatherers who obtained food by hunting and by searching for edible wild plants. They needed strong bones, including large, strong jaws that enabled them to eat tough, uncooked foods. When our ancestors developed agriculture, however, their diet changed. They began growing plants, such as grains and beans, and raising animals for food; they also started cooking their foods, making them softer and easier to chew. Consequently, the human skeleton underwent radical changes. Over time, the human jaw became smaller and changed shape. Other bones also evolved, becoming lighter, especially in the joints, as a result of both dietary changes and a less active lifestyle.

What is the central idea of the passage?

• As early humans developed agriculture, their bones changed significantly.

• Early humans ate uncooked meats and vegetables, so they needed strong jaws for chewing.

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April Fools' Days Around the World

April Fools' Day is celebrated around the world, although it varies somewhat from country to country. In the United States, for example, it's a day to play practical jokes. In France, where the holiday is known as Poisson d'Avril, or April Fish, people secretly tape paper fish on their friends' backs. A common April Fools' prank in Portugal is to throw flour on someone; in Scotland, the most popular trick involves sending someone on a useless errand. Denmark celebrates April Fools' Day on April 1, but some people there celebrate again in May. Maj-kat, on May 1, is yet another day for jokes and pranks in the spring.

What is the central idea of the passage?

• On Poisson d’Avril, France’s version of what Americas call April Fools’ Day, prakksters tape paper fish to people’s backs.

• Though there are variations in how different countries observe April Fools’ Day, the celebrations all involve playing pranks.

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The Importance of Japan's Cherry Blossoms

In the northeastern United States, autumn is particularly colorful. Leaves change from green to red and orange, and many Americans take scenic fall foliage trips. In Japan, a similar phenomenon takes place in the spring. Cherry trees erupt in a shower of pink, and the entire country celebrates the arrival of sakura, or cherry blossoms. Though autumn leaves are admired in the United States, they are seen as just a pretty seasonal display. In Japan, on the other hand, cherry blossoms have much greater significance; the flowers are deeply rooted in Japan's culture and philosophy. Since as far back as the eighth century, sakura have symbolized change and beauty, new beginnings, and the circle of life. The blossoms have been celebrated in Japanese paintings, poems, and tea ceremonies for centuries.

What is the central idea of the passage?

• Americans appreciate the colorful leaves of fall, but the Japanese view the cherry blossoms of spring as part of their culture.

• Japan’s spring cherry blossoms often serve as symbols of change or new beginnings in works of Japanese art or literature.

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