Why does boiling oatmeal "grow"? When cooking oatmeal (rolled oats, either long-cooking or instant) on the stove or in the microwave, when it reaches the boiling point, if left unchecked it will quickly "grow" and overflow the container in which it is cooking, even if it is a very large container.

A comparable amount of water boiling in the same pot or container would not do this.  It would boil, yes, but it would not "grow" and overflow the container.  I have not seen this happen with other grains.

Why does oatmeal do this?
Oatmeal swells when it absorbs water and is sticky enough to bind into a near-continuous mass. Water that is not actually absorbed into a grain is trapped by swollen soft, sticky goo. But it is still water, not chemically combined, and if you heat it to 100 C, it will volatilize with great expansion, stretching the goo as it does so, but the bubbles can't make it all the way out. 

In a way, it's like a slow motion popcorn, or a speeded-up rising of yeast bread. You can do something similar with a marshmallow.

I found that if you exactly repeat a recipe, the microwave timer could be precisely set to swell the oatmeal to fill the bowl, and not overflow. Then you let it cool down and do it again.