system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How were scientists able to discover the atom and its parts without being able to directly observe them?
gpt
Atoms as a concept were theorized by ancient Greeks, and the name literally means "indivisible." While the name was proven to be somewhat incorrect, the reasoning behind everything being discrete particles instead of continuous matter is old.

It wasn't until the 1800s until their existence was actually proven. A guy named John Dalton figured out that many chemical reactions always react in ratios of small whole numbers. Water, for example, consists of oxygen and hydrogen with exactly a 1:2 ratio, which suggested that the base reaction consists of one unit (or atom) of oxygen reacting with two hydrogen atoms.

Later on we devised more accurate ways of detecting them, like measuring minuscule changes in weight, shooting electrons at them or just crashing them into each other hard enough to produce interesting results.