How to do the age cards mind-reading trick

The age cards are a set of number cards, usually five or six. Someone tells you which cards show their age, and you name it at once. The secret is binary: each card starts with 1, 2, 4, 8, 16 or 32, and you just add the first numbers on the cards they pick. You can make a set in ten minutes with a pen and paper.

What does the audience see?

You hand someone a few cards covered in numbers. They look through them and give you back every card that shows their age. Without asking anything else, you tell them how old they are. It works for any age on the cards, and you can repeat it with another person straight away.

Why does it work?

Every whole number from 1 to 63 can be made by adding a unique set of the numbers 1, 2, 4, 8, 16 and 32, each used once at most. This is how binary numbers work: each place is worth twice the one before. For example, 47 is 32 + 8 + 4 + 2 + 1, and there is no other way to make 47 from those numbers.

The cards are built so that a number appears on a card only if that card’s power of two is part of it. So 47 is on the cards headed 1, 2, 4, 8 and 32, but not the card headed 16. Add the first numbers on the cards you get back and you have rebuilt the secret number.

How do you make the cards?

1. Choose your range. Five cards cover 1 to 31. Six cards cover 1 to 63. For ages, six is safer.
2. Head each card. Write 1 at the top left of the first card, then 2, 4, 8, 16 and 32 on the others. That first number is the only one you need to remember for each card.
3. Fill in the numbers. For each number from 1 to 63, break it into powers of two and write it on those cards. Some patterns help:
   • The 1 card holds every odd number.
   • The 2 card holds pairs: 2, 3, then 6, 7, then 10, 11, and so on.
   • The 4 card holds runs of four: 4 to 7, 12 to 15, 20 to 23, and so on.
   • The 8, 16 and 32 cards follow the same pattern with runs of 8, 16 and 32.
4. Check a few numbers. Pick 23. It is 16 + 4 + 2 + 1, so it should be on the 16, 4, 2 and 1 cards only.
5. Mix up the order on the cards if you like, so the pattern isn’t obvious. Just keep the power of two in the top left corner.

How do you perform it?

1. Hand over the cards and ask the person to give back every card that shows their age. Tell them not to say the number.
2. As each card comes back, glance at its top left number and keep a running total in your head.
3. Don’t answer straight away. Look at them for a moment, as if reading their face, then name the age.

A nice touch is to let them hand the cards back in any order. It makes no difference to the total, and it suggests you couldn’t be counting.

How do you make it feel like mind reading?

• Change the question. Ages are expected. A birthday date, a favourite number or the number of a hotel room feels more personal.
• Use it twice. Find their age, then their partner’s age, and then reveal both together.
• Go hands-off. Lay the cards on the table and have them point, so you never touch anything.
• Combine it. Our 1089 number trick and other number effects make a short, easy mind-reading set.

What are the common mistakes?

• Mistakes on the cards. Check every card once. One missing number and the trick fails for that person.
• Adding out loud. Keep the sum in your head. Moving your lips gives it away.
• Answering too fast. An instant answer looks like a calculation. A short pause looks like intuition.

Where does the trick come from?

We haven’t found a reliable earliest source for the age cards themselves. Number divinations have a long history in recreation books: Henri Decremps’ Philosophical Amusements (English edition, 1790) includes number tricks such as finding a number thought of, and W. W. Rouse Ball’s Mathematical Recreations and Essays (first published 1892) covers finding a chosen number among its arithmetical recreations.

What helps you take it further?

• Mathematical Recreations and Essays by W. W. Rouse Ball, free from our public-domain library. It collects arithmetical recreations, magic squares and card problems, and is a favourite source for number-based effects.
• Philosophical Amusements by Henri Decremps, also free. Number divinations, card tricks and optical experiments, each set out step by step.
• How to Read Minds Kit by Peter Turner. A beginner-friendly kit with over 20 techniques and special props, including how to reveal a word someone thinks of.

Browse more in our easy mentalism collection.

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