Cards on your mind – the secret

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:

     8 4 2 1
1    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.

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