Print out any one of the above pictures on a piece of paper or card. Ask a friend to pick any number within the circle.
Explain that you are going to point randomly at different numbers in the circle while he silently counts up to 20 in his mind. With the number he has picked earlier, ask him to start counting from the next number
and add one to the new number every time you point to a random number in the circle. And by the time he gets to 20th number ask him to say STOP and bingo ! You'll be pointing to his chosen number without fail.
Your friend picked the number 9, so he will start to count from 10 (in his mind)
when you point to the first random number in the circle, 11 when you point to the second random number in the circle, 12 when you point to the third random number in the circle and so on.
When he gets to 20, he should say, "Stop!", and you will be pointing to the number that he has picked earlier!
HOW TO DO IT
As your friend is counting to 20, you are also counting. The first seven number you point to
can be any number in the circle. However, the eighth (8th) number you point to must be 12. Then point backwards with the numbers (eg. 11,10,9,....) until your friend tells you to stop.
This is a table of reference on how your friend count and what you count.
(This table illustrates the counting reference in the example when your friend has picked the number 9)
Have fun with this free magic trick.