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 atoms and not just one?


How do we know that, for example, H2O, is infact H2O and not just one hydrogen? Further more, how do we know that anything is made of specifically these elements and not something else? And what makes molecules stay together (like in a human)?
gpt
You can take pure oxygen and pure hydrogen and combine them to form water. By measuring how much oxygen or how much hydrogen is left over after all the water is formed, you can deduce the proportions that were used to make the water. When we do this, we can tell that two parts of hydrogen are used for every one part of oxygen.

What makes molecules stay together are things called intermolecular forces, of which there are a variety and rather complicated to explain. But basically there are attractive forces between the molecules that keep them together rather than just flying apart.