A grain that drinks
Why Does Rice Need Exactly the Right Water?
Rice does not boil soft — it absorbs a fixed amount of water and then stops.
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Step 1 of 5
A dry grain is packed starch.
Cooking rice is the job of getting water into those bundles. The starch inside sits in tight crystal-like bundles. In that state it is hard and your body can barely digest it.
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Step 2 of 5
Hot water forces its way in.
Above about 60 degrees water can push between the starch bundles. They swell, soften and the grain grows to roughly three times its size.
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Step 3 of 5
Each grain takes a fixed amount.
A grain will absorb about its own volume and a half of water and no more. That is why rice is measured by ratio rather than cooked by eye.
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Step 4 of 5
Extra water has nowhere to go.
Once the grains are full, the leftover water sits in the pan and the outsides break down into it. That is what makes rice gluey.
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Step 5 of 5
Resting finishes the job.
Off the heat and lidded, the steam left in the pan spreads through and evens out the grains that got there first.
Rinsing rice first washes off loose surface starch, which is the other half of keeping the grains separate.
The short version
Rice cooks by absorbing a fixed amount of water, so the ratio matters far more than how long you boil it.
Try it yourself
Measure a cup of dry rice, then measure it again cooked. It will have roughly tripled.