Why does rice become sticky after cooking?
Starch is what makes rice sticky. Starch surfaces, when dry, are closely packed together and they do not stick to each other. If they get moist, especially if left to soak, or boiled, the starch will "unwind" as it absorbs water and become sticky. Depending on the source of starch and the type, you can have cleavage of starch to sugar and it will become even more sticky. You can read this pdf for more information

Funfact, you can make your own glue with flour and water as an example. 



I do not take responsibility for any mess or any unwanted results if for instance you decide to glue the broken baby seat with home made glue. =)