Our perception of our body shape is very malleable - making your finger feel shorter.

Here is a neat trick. It works! (I tried it). Ekroll et al. show that illusory visual completion of an object's invisible backside can make you finger feel shorter. Here is their summary and the central graphic from the article.

Highlights

•The experience of the hidden backsides of things acts as a real percept 

•These percepts have causal powers, although they do not correspond to real objects 

•They can evoke a bizarre illusion in which the observer’s own finger feels shrunken 

•The perceptual representation of body shape is highly malleable

Summary

In a well-known magic trick known as multiplying balls, conjurers fool their audience with the use of a semi-spherical shell, which the audience perceives as a complete ball. Here, we report that this illusion persists even when observers touch the inside of the shell with their own finger. Even more intriguingly, this also produces an illusion of bodily self-awareness in which the finger feels shorter, as if to make space for the purely illusory volume of the visually completed ball. This observation provides strong evidence for the controversial and counterintuitive idea that our experience of the hidden backsides of objects is shaped by genuine perceptual representations rather than mere cognitive guesswork or imagery.

Figure

A Well-Known Magic Trick and the Shrunken Finger Illusion

(A and B) The multiplying balls routine. The magician first holds what seems to be a single ball between his fingers (A). After a quick flick of the wrist, a second ball seems to materialize (B). In reality, the lower “ball” is a hollow semi-spherical shell, from which the real ball is pulled out.

(C and D) Schematic illustration of the shrunken finger illusion. When a semi-spherical shell is balanced on the observer’s finger as shown in (C) and viewed from above, the observer often reports perceiving the shell as a complete ball (D), while his or her finger is felt to be unusually short, as if to make space for the illusory volume of the complete ball. Note that this drawing is an exaggerated caricature of the perceptual experience. In particular, the real effect may be smaller than depicted here. In the experiments, only the middle finger was extended, while the other fingers were closed to a fist (see Figure below).