UK scientists believe they have identified the part of the brain that determines whether a person feels fat.
This area is called the posterior parietal cortex and is at the side of the head, just above the ear, and it processes sensory information from body parts.
The team from University College London Institute of Neurology say their findings might explain why some people feel fat, even when they are thin.
Their findings are based on brain scans of volunteers experiencing a body size illusion.
In the study in order to achieve the so-called "Pinocchio" illusion, the 17 participants each had a vibrating device placed on their wrist to stimulate the tendon and create the sensation that the joint was bending, even though it was not.
With their hand touching their waist, the volunteers felt their wrists bending into their body, creating the illusion that their waists were shrinking.
It seems that during the experiment, all of the volunteers reported that they felt as though their waist had shrunk by up to 28%.
The team believe that people who have problems with judging the size of their body might have a distorted representation of their body image in the parietal cortex.
Lead researcher Dr Henrick Ehrsson says the brain scans revealed increased activity in the posterior parietal cortex at the same time; this is the brain region that helps interpret the sensory information coming from different parts of the body.
The volunteers who reported the strongest shrinking sensation also showed the strongest activity in this area of the brain.