What Democritus Thought
(Around 400 B.C.)
In the days before electricity, the internet, cable TV and cell phones, people actually spent a lot of time thinking and philosophizing about the world around them (it's true - I was there!).
While the "ancients" believed everything was made up of either earth, wind, fire, or water, Democritus had other ideas. He believed - after hearing a guy named Leucippus chat - that there had to be a basic building block that made up all other matter. Common (and useful) analogy is to think of a big toy castle made only of blue logos. Each single blue lego would be an "atom," though Democritus called them "atomos."
Since Democritus really based his theory on observations and natural curiosity, "experiment" earns ye olde air quotes here. Going forward you will need to match the scientist with his or her theory and understand the experiment that supported it. For Democritus we sometimes just tag his experiment as "Big Thoughts."
Democritus believed the atom was it - smallest particle of matter. Case closed. Makes sense if you stop and think about it. He was already talking about tiny invisible things, and there was no technology available to him that would allow him to see the unseeable. I think it's pretty remarkable he was able to develop a good basic concept of atoms to begin with.
Would it not have been for Aristotle (I'm sure you've heard of him; big philosopher, pretty popular in his day) raining on Democritus' parade and writing atoms off as nonsense, the idea probably would have gained traction a lot earlier.