Finding an Implied Main Idea Study Guide (page 2)

Updated on Aug 25, 2011

Casting a Net

When you're looking for an implied main idea, what you're really doing is searching for the right "net" to cast over the passage. What is the idea that encompasses all the other ideas in the passage? What holds it together? (Remember, a paragraph, by definition, is a group of sentences about the same idea.)

What if you're looking for the main idea of several paragraphs? Well, it's really the same thing. Instead of determining the main idea of an individual paragraph, you're determining the overall main idea. Remember the comparison between a table and an essay? In an essay, the overall main idea is the tabletop, while the supporting ideas are the legs that hold up (support) the table. Each of those legs, though, might be paragraphs of their own with their own main idea and supporting sentences.

Here's a very short essay with an implied main idea. Read it carefully. Can you see what the whole passage adds up to?

It has been more than 25 years since the National Aeronautic and Space Administration (NASA) last sent a craft to land on the moon. A lunar prospector took off in January 1998, in the first moon shot since astronauts last walked on the moon in 1972. This time, the moon-traveler is only a low-cost robot that will spend a year on the surface of the moon, collecting minerals and ice.

Unlike the moon shots of the 1960s and 1970s, the lunar prospector does not carry a camera, so the American public will not get to see new pictures of the moon's surface. Instead, the prospector carries instruments that will map the makeup of the entire surface of the moon.

Scientists are anxious for the results of the entire mission and of one exploration in particular—that done by the neutron spectrometer. Using this instrument, the prospector will examine the moon's poles, searching for signs of water ice. There has long been speculation that frozen water from comets may have accumulated in craters at one of the moon's poles and may still be there, as this pole is permanently shielded from the sun.

Which of the following statements seems to best express the overall main idea of this passage?

  1. There is a great deal we can learn from studying the moon.
  2. The prospector will collect surface data rather than take pictures.
  3. NASA's newest moon-traveler is on an important mission.
  4. Scientists hope the prospector will return with evidence of water on the moon.

If you remember that a main idea must be general enough to hold the whole passage together and that a main idea must also be an assertion about the subject, then it should be pretty easy to tell which is the correct answer. First, choices b and d are too specific to be the main idea; they deal only with information in the second and third paragraphs, respectively. Second, they state only facts; they don't make a claim about the subject.

They can't be the overall main idea for this passage. Choices a and c, on the other hand, both make assertions about the subject and are general. Notice how they both allow room for detailed support. But while choice a casts a wide enough net, it's not the right net for this passage. The passage is about what NASA hopes to learn from this specific mission. So while choice b and choice d are too specific, choice a is too general to be the main idea of this passage. "NASA's newest moon-traveler is on an important mission," however, casts a net that's just the right size, thus choice c is the correct answer.

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