This is, as the kids say, cap. Popcorn works because the outer hull is strong enough to hold together until a critical mass of pressure builds inside to rupture it. At that point the starches, which have liquefied under pressure, explode and unliquify rather suddenly. White rice and pearled barley have their hulls removed. You can see in the picture that the barley has split, but not popped, and the rice just looks a…
Edit: If no one replies, I'll try tonight and report back by tomorrow morning.