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?