system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we know water is made of two hydrogen and one oxygen atom?

Curious about how scientists came to discover this, google didnt help much.

EDIT: thank you so much for the help!
gpt
You can separate the two elements with an electric current, a 9 volt battery is plenty. It is called electrolysis. Look it up and you can see how to make or buy a small apparatus to do it. Fill two test tubes with the resulting gasses and you can measure the volume of gas, showing that the hydrogen tube has twice the volume of the oxygen tube. You can test the properties of the gasses (explode the Hydrogen, use the Oxygen to make something burn brighter, etc.)