Let's open the course with a quiz. Relax — it's grade-school arithmetic:

A bat and a ball cost $1.10 in total. The bat costs $1.00 more than the ball. How much does the ball cost?

Don't scroll down yet — say your answer in your head within 3 seconds.

...

Got it? If what popped up was '10 cents' — congratulations, you're standing with more than half the students at Harvard, MIT, and Princeton: all wrong. One quick check exposes it: if the ball were 10 cents, the bat ($1.00 more) would be $1.10, totaling $1.20 — too much. The correct answer is 5 cents (ball $0.05 + bat $1.05 = $1.10). This is the first item of Frederick's (2005) Cognitive Reflection Test (CRT), and the single most famous question in all of 'intuition trap' research.

Why is almost everyone's first reaction 10 cents? Because two processes run in your head:

- System 1: automatic, fast, nearly effortless, impossible to switch off — like recognizing a friend's face at a glance;
- System 2: slow, serial, effortful, demanding motivation and attention — like computing √19163 without a calculator.

The labels were coined by Stanovich and West in 2000, then borrowed and popularized by Kahneman's Thinking, Fast and Slow (2011) as the framework of the whole book. The intuition trap lives exactly at their handoff: System 1 blurts out a fluent-sounding candidate ('$1.10 splits into $1 and 10 cents — so smooth!'), and System 2 often signs off without so much as a glance. So the CRT was never testing arithmetic — it tests whether you can hold back the answer that leaps to your tongue.