How Mind Reading Cards work

Each card has a secret: its smallest number. Add up the smallest numbers on the cards where you said yes, and you get your number.

Six numbers that make every number

The smallest numbers on the six cards are 1, 2, 4, 8, 16 and 32. Each is double the one before. Think of them as coins. Using each coin once at most, you can make any number from 1 to 63, in just one way. 42 is 32 + 8 + 2.

Each card holds the numbers that use its coin

The 8 card holds every number that needs the 8 coin. So 42 is on the 32, 8 and 2 cards, and no others. Your answers tell the site which coins your number needs. It adds them up.

This is binary, the way computers count: each yes is a 1 and each no is a 0, so 42 is 101010. The numbers on each card are jumbled, and the cards come in a new order every time, so nobody spots the pattern. A number outside 1 to 63 is on no card at all.

Do it for a friend

Make six cards. For the 4 card, start at 4, write 4 numbers, skip 4, and repeat up to 63. Do the same for 1, 2, 8, 16 and 32. Show them one at a time, and add the smallest number on each yes card.