In the ancient world, the metaphor for a brilliant idea just might have been a firecracker exploding over someone's head. That's because firecrackers contained black powder, the invention of 10th-century Chinese pyromaniacs. It didn't take long before some bellicose warrior or jealous husband discovered he could launch a projectile using the same mixture of saltpeter (potassium nitrate), sulfur and charcoal.
The earliest black-powder weapons belonged to the Arabs -- bamboo tubes reinforced with metal that used a charge of black powder to shoot arrows. These were replaced by bronze hand cannons, which required two men to fire. One held the weapon while a second inserted a glowing coal or wire into a hole drilled in the solid end, or breech. This ignited the black powder, which sent a round ball -- the first bullet -- roaring from the open end of the cannon.
Over time, weapons became far more sophisticated, but they still relied on the same ancient chemical process, what scientists describe today as deflagration. In this type of reaction, a spark ignites a small mass of black powder, which doesn't explode but combusts rapidly to create a large amount of expanding gases held back by a non-fixed plug. That plug, of course, is the bullet, which fits tightly enough in the barrel that gases can't escape around it. As the gases expand and encounter the resistance, they propel the bullet out of the muzzle.
It would be another nine centuries before something better came along.