Enter the maze

Tick-tock, a magic clock

Black and white image of a clock

In this quick magical effect you can predict the future and also read your friend’s mind at the same time. All you need is a pack of playing cards and you too can do magic with a clock face.


Take the deck of cards and decide which card you want to use for your prediction. Say we choose the 8 of hearts: put it in the pack, 13 cards down from the top. To start the trick write ‘8 of hearts’ on a bit of paper, fold the paper shut and put it down on the table. Put your deck of cards on the table (remember your prediction card is at position 13 from the top). Ask your friend to think of their favourite hour of the day. It could be 1 o’clock up to 12 o’clock, 12 possibilities, but only they know their choice. To make sure your friend doesn’t change their mind, give them the pack and get them to count off a number of cards equal to their chosen hour, for example, for 6 o’clock they would count off six cards. You turn your back (don’t peek) as they count off the selected number of cards and hide them in their pocket. Finished? You can turn round now.


You take the pack and count off 12 cards onto a pile; putting one on top of the other (this will inconspicuously reverse the order of the cards which is the secret of the trick). Take this pile of 12 cards and use them to create a clock face on the table. Starting at 1 o'clock arrange the cards just like a clock, face up in a circle. Then dramatically place the folded piece of paper with your prediction in the centre! Summon up all your mental powers, and in your head count round the clock face (handy hint – don’t move your lips), starting at 1 o’clock till you get to your prediction card. If your prediction card is at number 7 you know your friend has chosen 7 o'clock. Now you do a dramatic reveal: ‘I sense you chose…6 no wait 7 o’clock, is that right?’ They will agree, and then you get them to confirm by counting out the 7 previously hidden cards from their pocket. Result! That’s good stuff but there’s more to follow.

Big deal

They had a totally free choice, and chose 7 o’clock. But you now remind them of your previously placed paper prediction. Count round the clock face from 1 o’clock to 7 o’clock with them (this time you can move your lips) and there, at their freely chosen time, is the eight of hearts. Open your prediction and enjoy the astonishment on their face.

What's the deal?

This trick works because it uses a) simple maths and b) a series of familiar patterns. Let’s look at the maths first. At the start your prediction card is at position 13 from the top. Your friend removes X cards (where X can only be in the range 1-12 as it’s an hour of the day), so the chosen card is now at position 13-X from the top. To make it easier let’s look at a particular example where X= 4. After 4 cards are removed the chosen card is now at 13-4 = 9 from the top. You deal 12 cards while reversing the order, so it’s now at 12-9 = 3 cards down from the top. Now, when you deal these cards into a circular clock face starting at 1,2, and so on, the chosen card is now at the 4 o’clock position, just where you want it!

Looks familiar?

The rest is about hiding this simple maths in familiar patters, like a clock face. Everyone knows what a clock face looks like; it’s a natural pattern. The initial removal of the cards to hide them in the pocket is just a way to prove the selection at the end of the trick. Again this seems totally natural, but behind these day-to-day patterns the maths, and thus the magic, is hidden. Computer scientists also hide the complexity of their programs by making them as natural to use as possible. These are called intuitive interfaces.

Mind the gap

To get this right is actually quite tough. If you’re building software you need to think about what people know (often called current knowledge) and what they would have to know to use the software properly (this is called target knowledge). These can sometime be very different, leading to loads of frustration! It’s called the knowledge gap. Software designers always have to look at this. Will they design expecting that only current knowledge is needed, or will they try to train the user?

For software in everyday gadgets and applications like set-top boxes or word processors, programmers often just expect people to read a manual first, perhaps providing helpful design elements, like pop-up help. Sometimes that works. Other times it leads to people giving up and ignoring many of the functions designed into the software.

With more complex software for professional use, designers often expect people to go on a training course before they use the software. No pilot is allowed to fly a plane without hours of training on simulators for that plane for example! The assumption that people will be trained can be problematic though. Take nurses. They have to enter volumes and dose rates of drugs for their patients into pumps that deliver the drugs. Hospitals typically have many different makes and models of device all with their own subtly different interfaces, though. The best design for accident and emergency may not be appropriate for a cancer ward for example. The nurse is probably an expert with the device on their normal ward but what if they have to fill in somewhere else due to illness. Even if they were trained on the devices of that ward, it may have been a long time ago, so they may believe they can use it but actually be rusty. Their current knowledge may be different to the target knowledge needed. Wherever possible designers of medical devices therefore need to keep the knowledge gap small so nurses don’t waste time trying to work out what to do or undoing mistakes. Lives may depend on it!

Whatever the application, getting the knowledge gap right is often the difference between software success and failure – time and time again.