The origin of the idea of explosions

Everybody knows that the Chinese invented gunpowder, but the question is where did they get the idea for something that could explode.  One place where the idea of explosions could have come from is putting wet, green bamboo rods into the fire, bamboo comes in sealed off sections, the fire heats the bamboo with the moisture inside, the moisture heats up, turns to steam and expands and the bamboo pops.  It is not much, but it is a natural source for the idea of an explosion, which probably got alchemists searching how to replicate it.  

 Or of course, there is what we learned in elementary school, that if you don't get all the bubbles out of poverty before firing it, it explodes and wrecks everybody else's homemade ashtray or candle holder.  But that must be much later than putting bamboo on a fire.