Goal

Put each card on one of the four piles. A hidden rule decides the right pile — matching by colour, shape or count. You're never told the rule: a gold tick or a red cross after each card is all you get. Work it out, and stay with it.

The rule changes

• After a run of right answers the rule changes to another one. Levels 1–2 tell you; from level 3 it changes without a word — a cross where there used to be a tick is your clue.
• Crack 2 rules at a level to move up; 5 misses in a row move you down.
• Higher levels make the runs shorter and less predictable. From level 6 the piles change places when the rule changes; at level 8 each card has 2.5 seconds.

Scoring

A right card earns 1 + the level; finishing a rule's run earns 3 more. A miss costs 1 + half the level — finding a new rule takes a miss or two, but sorting at random never pays.

Set-Shifting

This is the idea behind the Wisconsin Card Sort, the best-known test of mental flexibility: noticing that what used to work has stopped working, and letting go of it.

Learning from Feedback

Every tick and cross is evidence. The end card counts how often you stayed with an old rule after it changed — a number that drops as you get quicker to adapt.