What happens when rice is cooked?

Uncooked dry rice grains have some moisture, but not a lot. The usual rice you buy in stores, both long grain and short grain, has about 12% moisture content, meaning each grain is 12% water by weight. Aged rice has lower moisture content. If you buy aged Basmati rice, for example, the moisture may be as low as 7-9%, depending on how long it was aged.

Cooking does two things to the rice. First, it hydrates the rice grain (adds moisture to it), making it swell and increase in size. Second, the heat cooks the rice grains, making them soft and slightly gelatinous.

Properly cooked rice has a moisture level between 58% and 64%. Anything below that range will feel hard and uncooked and anything over that range will be mushy. You may prefer one end of the range rather than the other, depending upon the type of rice and your personal tastes. So the goal of cooking is to increase the water content of the rice from 12% to about 60%, and also heat the interior of each grain for a sufficiently long period that it is cooked.