Odd or big?
A number with a cue: ODD? or BIG? (more than 5). Answer yes or no for the rule shown; the rule keeps changing. More clashes, less time.
How it plays
A number appears with a question beside it: ODD? or BIG? — big means more than 5. Answer yes or no for that question only. The question changes without warning from one number to the next.
Each rule is easy. The hard part is the switch, when the rule you just used gives the opposite answer for this number: 7 is big and odd, but 3 is odd and not big. Levels raise the share of numbers where the rules clash, and shorten the time, which closes on 0.7 s.
The "run" format shows a column of cued numbers to answer at once (type Y or N for each), where the levels also set how often the rule switches. A third rule, PRIME?, is a setting.
Keys
Space starts and skips ahead · 1–9 pick · Enter submits · Esc stops. On a phone, tap.
What it trains
This practises switching between two simple rules without letting the old one answer. It improves this task; it is not, on the evidence, a route to general mental flexibility.
Learn
How to get better at odd or big?: the method, the techniques, what each new level asks of you, and the traps the wrong answers are made of.
Your approach to each question
- Read the cue word first — ODD?, BIG? or PRIME? — and only then the number.
- Ask only that question of the number. BIG? means more than 5; 5 itself never appears.
- Press 1 for Yes or 2 for No.
- In the run format, work down the column one line at a time, cue then number, and type Y or N for each before you read the next.
Techniques and tricks
The four boxes
The numbers are 1, 2, 3, 4, 6, 7, 8 and 9. Learn them as four pairs: small and odd 1, 3; small and even 2, 4; big and odd 7, 9; big and even 6, 8. Each number then carries both answers with it.
Know the clash numbers
With ODD? and BIG?, the two rules agree on 2, 4, 7 and 9 and disagree on 1, 3, 6 and 8. On a clash number the old rule gives the wrong answer, so that is where to take a beat.
e.g. 3 is odd but not big; 8 is big but not odd.
Say the cue in your head
Read the cue word to yourself as you look at the number — “odd?… seven” — so the question you answer is the one on screen, not the one from before.
PRIME? is only three numbers
Among the numbers this game shows, only 2, 3 and 7 are prime. 1 is never prime, and 9 is 3 × 3. Hold the three as a set.
e.g. PRIME? 9: no. PRIME? 2: yes, though 2 is even.
Mark the switches in a run
Before answering a column, glance down the cue words and notice where they change. A switch you saw coming costs less than one that catches you.
As the levels climb
- With one number at a time, the share of numbers where the rules clash rises from half to 70% at level 2 and 85% at level 4. From level 4 most numbers are clash numbers, so the four boxes have to be instant.
- The clock starts at 3.5 s and shrinks at levels 1 and 3, then every level after level 4, closing towards 0.7 s without reaching it.
- In the run format the column grows a line every few levels from three, the chance of a switch from one line to the next rises from 30% to 50%, 70% and 85%, the clashes rise as above, and the time tightens to 62%.
Traps to avoid
- The stale rule: BIG? 3 answered Yes because the last question was ODD? and 3 is odd. The working names the rule that “would have said” the wrong answer.
- Big and even: ODD? 6 or ODD? 8 answered Yes, because both are big. With ODD? and BIG?, these two and 1 and 3 are the whole of the trap.
- With PRIME? on, 2 answered No because it is even, or 9 answered Yes because it is odd.
- In a run, typing one letter too few or too many. A typed run earns credit for each letter in its place, so a skipped line throws off every line after it.
How to practise
- Once single numbers are steady, set “Format” to “A run of numbers, typed Y or N”, where the levels also raise how often the rule switches.
- In “Rules”, tick “PRIME? — is it 2, 3 or 7” for three rules: now a number can clash with two rules at once.
- Set “Time pressure” to “Strict — 0.7× time” once you are at the top clash share, to make the four boxes automatic.
Levels
Each question has a clock that comes from the work it asks for. There is no last level: at least one dial turns without end. Your level is judged on your last ten answers, every ten: 90% moves you up, below 80% moves you down, which holds most people near four in five right.
All of it is in the settings beside Start — how many answers a judgement looks at and how often it recalibrates, the thresholds, the time, which questions, how many options, and whether you see each verdict at once or review them together at the judgement. You can also jump to any level.
More attention and speed
All games →Where, not what
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A board of symbols, a blank, the board again with one cell changed: tap it. More symbols, shorter looks and subtler changes as you climb.Symbol code
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