As the story goes, sailors in the British Royal Navy once took to mixing a tiny bit of gunpowder with liquor and trying to ignite it to determine if it was up to snuff.

If the powder didn't ignite, then the ration was too watered down. However, if it reacted, then they had "proof" that their alcohol hadn't been cut with too much water.

While history may not corroborate the tales of British sailors, the idea does seem to have been an actual practice when higher-taxed "proof spirits" first came onto the scene.

However, this was only before the invention of the hydrometer, a tool that measures alcohol content more accurately than volatile gunpowder. It was invented around 1730.

By the time rum became a staple aboard naval vessels around the early 1800s, not only was the hydrometer present aboard ships but its efficacy was being improved.