Simple Optical Illusion

Here is a simple optical illusion. Just a few ovals and a gradient makes this happen. It is hard to comprehend but easy to build. 

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Draw an oval, no fill color.

Select the oval and choose Object > Path > Offset Path. Set the Offset to -25 and click OK. 

This will create an oval smaller than the original.

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Select the two ovals and duplicate them so the bottom of the outside oval touches the bottom of the inside oval

This will create two sets of ovals. 

Select all the ovals and choose the Shape Builder Tool (Shift + M). Drag the Shape Builder Tool over the sections 2, 5 and 4. 

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Then drag the Shape Builder Tool over the sections 1, 6 and 3. 

This will create 2 shapes that form the optical illusion. 

Apply a gradient to one of the shapes. 

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Apply a gradient that is reversed direction to the other gradient. 

Add a heavy stroke to the shapes. 

Edit the colors in the gradient to change the color of the gradient. That simple!