Gunpowder - No one knows exactly who invented gunpowder.  All we know is it was invented by a Chinese alchemist (or perhaps several).  Like most alchemists, he was searching for the elixir of eternal life and wound up with a chemical that exploded.  The first references to gunpowder are found in records from the Song Dynasty.  It appears to have been well-known by then, so it likely was invented toward the end of the Tang.  Its military application wasn't realized for several centuries, and it was initially used only for entertainment purposes because of the dazzling explosions it produced.  Eventually someone discovered the explosions could be used to hurl a projectile toward an enemy at high velocity.  Humans never could have imagined at the time that the invention of gunpowder would be the first step in eventually hurling future generations into space.