Is it an optical illusion? Just move your head or eyes side-to-side and it looks like the bars in the circle are moving. This illusion is called Ouchi. You can use any of the illusions on Eluzions to engage your teachers or students.
How about adding one of the optical illusions to your website as an activity? Change the illusions randomly so your teachers or students will want to see the next puzzle.
Optical illusions were exploited in:
optical art, by artists such as Escher, and by Dali.
The variation in the apparent size of the Moon — it seems larger when near the horizon is an example of a natural illusion that is also an optical phenomenon.
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