How does boiled rice act as gum?

 Rice contains starch, a polysaccharide made of amylose and branched amylose amylopectin. In rice, it takes the form of granules which are clustered together to form compound grains. The dried starch granules when soaked in water swell and continue to do so when temperatures are is raised. Above a certain temperature the swelling becomes irreversible and the starch is gelatinized.

Lower the amylose content of starch, greater is its swelling. As rice starch has comparatively low amylose, it swells to a large extent on heating. This boiled rice when crushed becomes a paste which can be used as an adhesive.