If you cross your fingers and touch a small object (such as the tip of your nose), there will seem to be two of whatever it is. Not being able to see the object strengthens the illusion, and because you can’t see the end of your nose very well it is a suitable tactile target. Besides, using your nose is amusing.
This illusion has been known for at least two thousand years. Aristotle wrote (Metaphysics Book 4):
“… touch says there are two objects when we cross our fingers, while sight says there is one”
It’s an example of a tactile illusion.
Lots of optical (alias visual) illusions are known. They startle and intrigue: some seem almost incredible. Some have been discovered or invented in the last decade, others go back centuries. They all cast light on visual perception, and are all, even the most well known, still the subject of research and often controversy as well.
Tactile illusions are less well known and most people are only aware of the crossed fingers one. But many have been discovered and written about, and new ones emerge quite regularly. There is an excellent survey here.
One I particularly like is the ‘salad bowl after effect’. Take a smoothly concave bowl (like a salad bowl) and press three fingers – there is no need for it to be hard – against the inside curve of the bowl for a few seconds – perhaps 10 – and then touch them on a flat surface. If you are like me it will feel convex, as though a bump has risen up! This strange (and to my mind eerie) feeling only last a short time but for me at least its is quite strong. I’ve even found I can make it work with the inside of my glasses case.(Written some years ago for the Partialinsight blog.)