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Size Perception

In this demonstration, students learn about the importance of context in our brain's ability to interpret information.

In this illusion, one circle seems small compared to its large neighbours and the other seems large compared to its small neighbours. This happens because we hardly ever look at objects by themselves: our brains automatically seek to compare them to surrounding objects in order to determine their size. When a coin is placed in the middle of each circle, the brain is able to directly compare their sizes, dissolving the illusion.

This illusion can be used to the benefit of people who want to reduce their portion size when eating. Eating on a small dinner plate gives the illusion that we are eating more food than if it is served on a large dinner plate.


  • Explain that the brain makes its interpretations based on past experiences.


  • Per Student or Group:
    circles template, printed on either an overhead sheet or on a large sheet of paper overhead projector (optional)
    2 loonies or toonies

Key Questions

  • Which middle circle is larger?
  • What made you think that the circles were different sizes?

What To Do

  1. Show the students the circle image.
  2. Poll the students through raised hands: Which circles is larger? Smaller?
  3. Hold two large coins (loonies or toonies) and hold them in the middle of each circle to demonstrate the reality of perspective.


  • Challenge students to create their own size illusion.
  • How could this illusion help a person who is trying to control their portion size?