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Experiment Category: 

Objective: 

This is a trick that you can play on your friends which demonstrates that people see what they expect to see.

What You Need: 

  • A printer to print out the special cards
  • Scissors

To Do and Observe: 

A. Setting up 1. Print out the page that has the three cards on it. http://www.tryscience.org/experiments/denver/Opticalcard.pdf 2. Cut out the three cards. Be sure to cut out the windows in card 2 and card 3. B. Doing the Trick 1. Tell your friend that you will cast a magic spell so that they will not be able to read correctly! Say the magic words, "Canticus Readicus". 2. Show card 1 to your friend. Ask him or her to read the card. (Chances are high that your friend will answer either "B" or "13".) 3. After they answer, say, "Nice try. But let me give you a hint." 4. If they said "13", put card 2 in front of card 1 as in the diagram. Ask your friend to read the whole thing. Now they will probably say, "A, B, C". In other words, the same symbol that they read as "13" a moment ago, they are now reading as "B". 5. If your friend answered "B", then put card 3 in front of card 1 and have them read it. Their answer will probably be, "12, 13, 14".

What's Going On: 

Very often, people see what they expect to see, or hear what they expect to hear. Why is this? Your brain gets all of its information about the outside world through your five senses. But that is not enough info to put it all together. So your brain makes "best guesses" based on experience and other things. Over time it has come up with "rules of thumb" to help make these best guesses. One of the rules is to look at the bigger picture. "A B C" makes a lot more sense than "A 13 C". Your brain expects to see a B in between A and C and not a 13 and so, barring information to the contrary, that’s what it sees.

Parent/Teacher Tips: 

You can have lots of fun looking at optical illusions with your kids. Talk about how they work. What rules of thumb must your brain be using (or mis-using) in order to make those optical illusions work? Check out the cool web site: http://psylux.psych.tu-dresden.de/i1/kaw/diverses%20Material/www.illusionworks.com/index.html. Not only does it have excellent optical illusions, but it explains why many of them work.