TACHS: Main idea in a science passage: name the process and its limits
Learn to find the main idea of a how-it-works passage by labeling each paragraph, and to sort wrong choices into too narrow, too broad and contradicted.
How do you pick the main idea of a science passage when every choice mentions the topic?
Short answer
Give each paragraph a short label, then state the process that connects them, including any limits the writer adds. The answer matches that whole process without stretching it.
Know first: Reading a short informational passage, Telling a detail from a general statement
Step 1 of 5
The main idea is the process every paragraph explains
The main idea is the point the whole passage develops. In a science passage, that point is usually a process: what happens, what causes it, and what limits apply.
Many science passages open with how something looks and then explain what really happens. When you see may look or seems, expect a correction. The main idea is the corrected version.
| Choice type | What it does | Example clue |
|---|---|---|
| Match | Covers the process the whole passage explains | Uses the passage's cause and result |
| Too narrow | States one true detail from one paragraph | A fact you could underline in a single sentence |
| Too broad | Claims more than the passage says | every, always, exactly, all |
| Contradicted | Repeats the belief the passage corrects | Echoes the may look sentence |
Most wrong main idea choices fit one of the last three rows.
Step 2 of 5
Label each part, then connect the labels
Walking dunes
A sand dune can look like a permanent hill1. Wind, however, lifts grains from the dune's gentle side and drops them over the steep side2. Grain by grain, the whole dune creeps forward3. How fast it moves depends on the strength of the wind and the size of the grains, so two dunes in the same desert may travel at very different speeds4.
- The belief the excerpt is about to correct.
- The cause: wind moves sand.
- The result.
- The limit: speeds vary.
Worked example: What is the excerpt mainly about?
Which statement best expresses the main idea of the dune excerpt?
- ADunes are permanent hills shaped by a single storm.
- BWind gradually moves sand, so dunes travel over time.
- CAll dunes in a desert travel at the same speed.
- DSand grains come in different sizes.
- 1
Label the parts
Looks permanent. Wind carries grains over. Dune creeps forward. Speed varies.
- 2
Connect the labels
Wind moves sand grain by grain, and that makes dunes travel, at different speeds.
- 3
Sort the choices
B covers the cause and the result. A is the belief the excerpt corrects. C breaks the limit with all. D is a true detail used only to explain speed, so it's too narrow.
Answer
Wind gradually moves sand, so dunes travel over time.
Find the wrong step
A student answers the same question about the dune excerpt.
- 1
Reads all four sentences of the excerpt.
- 2
Notices that grain size is mentioned and that it is a precise, true fact.
- 3
Picks Sand grains come in different sizes because it is definitely stated in the excerpt.
What went wrong
Being stated isn't enough. That fact appears once, to explain speed. The main idea has to cover the whole excerpt.
The fix
Ask whether the choice covers every part. Only Wind gradually moves sand, so dunes travel over time connects the cause, the movement and the varying speed.
Step 3 of 5
Find the main idea and the role of each part
Read this excerpt
Many people think autumn paints leaves yellow and orange. In fact, those colors were in the leaf all summer. Green chlorophyll, which the leaf uses to make food, simply covered them. As the days shorten, the tree stops making new chlorophyll, and the green fades, so the hidden colors show. How bright they become depends on the weather, so the same tree can look different from one fall to the next.
Check yourself · Question 1
Which statement best expresses the main idea of the excerpt?
Answer: D
The excerpt corrects a belief, explains that the colors were there all along, and describes the green fading away. The right choice connects all of that.
Trap answer: Chlorophyll helps a leaf make its food.
True detail, too narrow
- Why it looks right
- It's a fact straight from the excerpt, so it feels safe.
- Why it's wrong
- It explains only why the leaf is green in summer. It says nothing about fall colors, which is what every sentence builds toward.
- The right answer
- Fall colors appear when green chlorophyll fades and uncovers colors already in the leaf.
Check yourself · Question 2
What is the purpose of the first sentence of the excerpt?
Answer: C
The next sentence begins In fact and says the colors were there all summer. The first sentence sets up a belief so the excerpt can correct it.
Common mistake
I chose the most interesting fact in the passage.
- Why it's tempting
- A surprising detail sticks in your memory, and it is true, so it feels important.
- Do this instead
- Test it against every paragraph. If a choice describes only one sentence or one paragraph, it's a detail, not the main idea.
Step 4 of 5
What to remember
Remember
- Label each paragraph in a few words, then state the process that connects the labels.
- When a passage says something may look one way, the main idea is what really happens.
- Cross out choices that are one detail, that break a limit with words like every or exactly, or that repeat the corrected belief.
Step 5 of 5
Practice on a real question
Use what you just learned on this question, then check the explanation.
In your own words
Write a short label for each paragraph of the practice passage. Which wrong choice states a single detail, and which one breaks a limit the writer set?