Roll the two pieces into a tube. I found mine worked best when I had a tube that was about 2.5 cm wide at one end and around 1.5 cm wide at the other.
Sticky-tape the tube so that it will not unroll.
Hold the wide end of the tube up to your left eye and look through it.
Raise your right hand up so it is next to the far end of the tube.
This can be the tricky bit: Uncross your eyes as though you are looking at a distant object.
It should look as through there is a hole in your hand.
What's happening?
Your brain was mixing together the images from each eye.
Because our eyes are in different positions, our eyes see slightly different images. Our brains combine the images we receive from each eye to produce our three dimensional view of the world.
This is what each eye sees.
When you looked down the tube, your left eye saw a small dot surrounded by darkness (we used two pieces of paper and made the tube narrower at one end to ensure the inside of the tube was as dark as possible). Your right eye was seeing your hand. When your brain mixed the images together, it appeared as though there is a hole in your hand.
When your brain combines the images from each eye, it looks as though there is a hole in your hand. It's not possible to photograph it, but this is how it appeared to me. This image was made by combining the two images with a computer.
Look down the tube with one eye and look at your hand with the other.
