How to play
There are 57 round cards with eight pictures each. The trick: any two cards share exactly one picture, never more, never less (the sizes and angles change, to make it harder).
You have your own card, and there’s one card in the middle. Everyone looks at once. Find the picture your card shares with the middle card and tap it, on either card. Be first and the middle card is yours: it becomes your new card, and the next one is turned up. Tap a wrong picture and you’re out for two seconds.
The quick game turns up 26 middle cards; the long game uses the whole deck. Whoever won the most cards wins.
Tips
Don’t search picture by picture. Look at the middle card as a whole, then glance at your own: the match often jumps out. Your card changes every time you win, so keep your eyes moving.
Why it always works
The deck is built from a bit of maths called a finite projective plane of order 7: 57 points and 57 lines, where any two lines meet at exactly one point. Each card is a line and each picture a point.