An illusion is a distortion of the senses, revealing how the brain normally organizes and interprets sensory stimulation. Though illusions distort reality, they are generally shared by most people. Illusions may occur with any of the human senses, but visual illusions (optical illusions), are the most well-known and understood. Illusions go a long way back! One of the oldest ever described dates back over 2,000 years ago, where Aristotle described an illusion that you can still take part in: If you cross your index and middle finder and then touch an object – like a pen – with both fingers crossed, it creates the illusion that you’re touching two pens at once!