Here’s a little trick you can play on your friends, or someone you don’t know well enough to already know their birthday…

With the five cards below, you can “guess” anyone’s birthday. Just go through each of the cards in turn and ask them if their birthday (as in the date they were born, not the month, so if they were born on the 17th January, their number is 17) is on the card. Discount the cards their birthday is not on.

With the remaining cards, the cards their birthday is on, add up the numbers in the top left corner and the number you get should be their birthday!

For example, my birthday is the 30th April so 30 in my number. Its on card 1,2,3,4 and not card 0 so you would add up 2+4+8+16=30.

### Is it science or is it magic?

Of course it’s science! This actually works on a system called binary, which is the language computers use. Binary is written in 0s and 1s and these together look just like 101001010010010101010 to us but to a computer that might actually mean something.

In this case, when you discount a card, that becomes a 0 and the remaining cards are a 1. So going back to the example of my birthday the cards would read 11110 (reading it backwards) and in binary this means 30.

There is at least one typo: on card 0 should appear 19 instead of 18.