Can something exist and not exist at the same time?

More generally, quantum mechanical uncertainty allows physical things to both exist and not exist at the same time. All that can be, is, in the measure that it can be, and for the rest it is not.

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Also, do we have free will?

At least since the Enlightenment, in the 18th century, one of the most central questions of human existence has been whether we have free will. In the late 20th century, some thought neuroscience had settled the question. In this context, a free-willed choice would be an undetermined one. How can our eyes deceive us? Our work shows that the cells of the primary visual cortex create small distortions, which then pass on to the higher levels of the brain, to interpret as best it can." This is one reason why people's eyes sometimes mislead them when looking at objects in their visual landscape.

Keeping this in consideration, what do illusions do to the brain?

When we experience a visual illusion, we may see something that is not there or fail to see something that is there. Because of this disconnect between perception and reality, visual illusions demonstrate the ways in which the brain can fail to re-create the physical world. Can the brain be tricked? New research shows that your brain can be tricked into thinking that the body is working harder than it is - just by listening to fast heartbeats. The human brain is complex, but it can be tricked. They conclude that it's possible to trick the brain into thinking that the body worked harder.

How your eyes trick your mind?

The basis of optical illusions is visual deception. It isn't your eyes playing a trick on you. Your eyes send signals to our brains through the retina, your brain then registers the information to create the image you are seeing. In the case of a visual illusion, the image the brain perceives differs from reality.