Level: B2 (Upper-Intermediate)
Content Objective: Students learn how habits form in the brain, why the “21 days” myth is inaccurate, and what research actually shows about habit formation.
Language Objective: Students practice modals of speculation (must, might, may, could, can’t + verb / + have + past participle) to express degrees of certainty.

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1. Reading Section

Why We Can’t Just Snap Out of It: The Psychology of Habits

Have you ever driven home and then realized you don’t remember the actual drive? That strange experience might tell us more about how the brain works than we usually assume.

For decades, psychologists have studied why some behaviors become automatic while others always require effort. The answer seems to lie in a small, ancient part of the brain called the basal ganglia. When you first learn a new skill — driving, typing, making coffee — your prefrontal cortex, the part of the brain responsible for conscious decisions, works hard. Every step must be planned. But once you repeat the behavior enough times, something shifts. The basal ganglia gradually takes over, and the behavior becomes automatic. This must be why experienced drivers can hold a conversation while merging onto a highway; their brains are no longer consciously managing the driving itself.

Researchers describe this process using a simple model called the habit loop. It has three parts: a cue, a routine, and a reward. The cue might be a location, a time of day, or even an emotional state. The routine is the behavior itself. The reward is whatever satisfaction follows, and it’s what teaches the brain to repeat the pattern next time. Once this loop has been repeated often enough, the habit could become almost impossible to consciously notice, which is part of why habits can feel so hard to break.

You may have heard that it takes twenty-one days to build a new habit. This idea has been repeated so often that many people accept it without question. But researchers could not find solid evidence for it. One influential study, led by psychologist Phillippa Lally at University College London, tracked participants trying to build new habits, such as drinking a glass of water at lunch or going for a short run. On average, it took 66 days for the behavior to feel automatic. For some participants, it took over 250 days. The twenty-one-day idea, it turns out, must have come from somewhere else entirely — some trace it back to a 1960s book about plastic surgery patients adjusting to their new appearance, not to any real study of habit formation.

Why does this matter? If a habit hasn’t formed after three weeks, that doesn’t mean something has gone wrong. It might simply mean the brain needs more repetition. The behavior could still become automatic; it just needs more time and more consistent cues.

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2. Vocabulary Section

Word / Phrase	Meaning	Example
automatic	happening without conscious thought or effort	Tying your shoes becomes automatic after enough practice.
conscious	aware of, and deliberately controlling, one’s own actions	Driving requires conscious effort at first.
ancient	very old (here: an evolutionarily old brain structure)	The basal ganglia is an ancient part of the brain.
influential	having a strong effect on people’s ideas or actions	The study became very influential in psychology.
participant	a person taking part in a study or activity	Some participants took over 250 days to form a habit.
consistent	staying the same over time; regular	A consistent cue helps a habit form faster.
trace back (to)	to find or identify the origin of something	Some experts trace the “21 days” idea back to a 1960s book.
assume	to accept something as true without proof	Many people assume the 21-day rule is scientific fact.

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3. Example Sentences (Target Grammar in Use)

• The habit must have already become automatic, since he wasn’t thinking about it at all.
• This might explain why some people quit new habits after just a few weeks.
• The twenty-one-day idea could have come from a completely different source.
• She may have read an outdated version of the theory.
• Researchers couldn’t have known the “21 days” claim would become so popular.

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4. Grammar Section

Modals of Speculation

We use certain modal verbs to guess or speculate about something — to say how certain we are, without stating it as a fact.

Modal	Level of Certainty	Present/General	Past
must	strong certainty (positive)	The theory must be wrong.	The idea must have come from somewhere else.
might / may / could	possible, but not certain	The cue might be a specific time of day.	She may have read the wrong study.
can’t / couldn’t	strong certainty (negative)	That can’t be true.	It couldn’t have come from real research.

Formation for past speculation: modal + have + past participle
Example: He must have practiced every day for the habit to feel automatic.

Quick Tip: Choose your modal based on how confident you are, not on what actually happened. Must = “I’m almost sure.” Might/may/could = “It’s possible.” Can’t/couldn’t = “I’m almost sure this is NOT true.”

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5. Comprehension Questions

A. True or False

1. The prefrontal cortex controls habits once they become automatic. (True / False)
2. The habit loop has three parts: a cue, a routine, and a reward. (True / False)
3. Research proves that habits take exactly 21 days to form. (True / False)
4. Phillippa Lally’s study found that habits took an average of 66 days to form. (True / False)

B. Short Answer

5. What part of the brain takes over once a behavior becomes a habit?
6. What are the three parts of the habit loop?
7. According to the reading, where might the “21 days” idea have originally come from?
8. What does it mean if a habit hasn’t formed after three weeks, according to the passage?

C. Complete with a modal of speculation (must / might / could / can’t + have + verb)

9. The twenty-one-day idea ______ (come) from a source unrelated to habit science.
10. He never thinks about tying his shoes anymore. That ______ (mean) it’s become automatic.
11. She quit after two weeks, so the habit ______ (not / have) enough time to form yet.
12. Rewrite using a modal of speculation: “It’s possible that the cue is an emotional state.” → The cue ______ ______ an emotional state.

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Answer Key

Click here to check your answers

A. True or False

1. The prefrontal cortex controls habits once they become automatic. — False (the basal ganglia takes over; the prefrontal cortex is active during the learning stage)
2. The habit loop has three parts: a cue, a routine, and a reward. — True
3. Research proves that habits take exactly 21 days to form. — False (researchers could not find solid evidence for this)
4. Phillippa Lally’s study found that habits took an average of 66 days to form. — True

B. Short Answer

5. What part of the brain takes over once a behavior becomes a habit? — The basal ganglia.
6. What are the three parts of the habit loop? — A cue, a routine, and a reward.
7. According to the reading, where might the “21 days” idea have originally come from? — Some trace it back to a 1960s book about plastic surgery patients adjusting to their new appearance, not to any real study of habit formation.
8. What does it mean if a habit hasn’t formed after three weeks, according to the passage? — It doesn’t necessarily mean something has gone wrong — it might simply mean the brain needs more repetition and more consistent cues.

C. Complete with a modal of speculation (must / might / could / can’t + have + verb)

9. The twenty-one-day idea must have come from a source unrelated to habit science.
10. He never thinks about tying his shoes anymore. That must mean it’s become automatic. (present speculation about a current state — no “have” needed, since “must mean” describes now, not the past)
11. She quit after two weeks, so the habit can’t have had (or couldn’t have had) enough time to form yet.
12. The cue might be an emotional state. (present speculation — matches “is” in the original sentence, so no “have”)