Vanessa Hill has shown a simple and brain-bending optical illusion on her YouTube channel BrainCraft that makes a "hole" emerge in the palm of your hand.
All you need is a narrow tube made out of rolled-up paper. Place your hand roughly 5 centimeters (2 inches) in front of one of your eyes, while you look down the tube with the other. For at least 10 seconds, try to focus on the sight in the distance through the tube and not your hand. After a while, it will appear that you can see through a hole in your hand.
As Hill explains in the video, it’s all to do with a phenomenon called binocular rivalry. Our brain uses the two images it receives from each eye to produce three-dimensional vision. However, if the two images present two very different sights, the brain will suppress one of the images and make the other more dominant – as opposed to creating a totally merged hodgepodge of the images.
In this case, the effect tends to suppress the hand as it’s the weaker stimulus and not in focus. However, it doesn’t just “black out” the image. Instead, the brain flickers between the images, creating the perception that the sight at the end of the tube is on your hand.
Check out Hill’s video to learn more about the optical illusion.