## This is the secret of the mind reading trick ‘Cards on your mind’.

## It relies on the special code called **binary** that computers use to store numbers. It is a way of writing numbers using only 1s and 0s. Each number is represented by its own sequence of 1s and 0s. Here are the numbers from 1 to 15 with their binary version:

84211 0 0 0 1 2 0 0 1 0 3 0 0 1 1 4 0 1 0 0 5 0 1 0 1 6 0 1 1 0 7 0 1 1 1 8 1 0 0 0 9 1 0 0 1 10 1 0 1 0 11 1 0 1 1 12 1 1 0 0 13 1 1 0 1 14 1 1 1 0 15 1 1 1 1

## You can use any two things instead of 1s and 0s. In the trick eyes open are used for 0 and eyes closed are used for 1. The secret number is then being spelled out in binary by the four people.

## You don’t have to remember the table to work out what number a binary code stands for. We put the numbers 8, 4, 2 and 1 above the columns. You just add up the column numbers to get the number. If there is a 1 in the binary in a column then that column number is added to the total. That is exactly what you do in the trick.

More magic of computing

More magic of computing