In the 1987 miniseries The Ring of Truth: An Inquiry into How We Know What We Know, Philip Morrison proves that a water molecule is composed of two hydrogen (H) atoms & one oxygen (O) atom with a simple demonstration. And you can prove it, too. With a regular carbon-zinc 9-volt battery attached to two stainless-steel screws and wrapped in oil-based modeling clay like Sculpey or Plasticine, you can break water into its elemental components.
These Science Snack Activity instructions from the Exploratorium explain how to create your own DIY electrolysis device. There’s also a combustion test at the end of the experiment:
Both oxygen and hydrogen gases are clear and odorless. So how do you know which test tube contains which gas? Here’s a clue: One filled faster than the other. There are twice as many hydrogen atoms available to form a gas, and thus the volume of hydrogen gas that forms should be greater than that of the oxygen gas.
The splint test gives another clue: Hydrogen gas is very flammable—a fact made famous by the Hindenburg zeppelin disaster—and makes an explosive popping sound when lit. Oxygen, on the other hand, is not actually flammable, but it is necessary for combustion, which is why your split relit in oxygen gas.
As always, be safe, smart, and do not conduct fire experiments without eye protection and an adult present.