Democritus was a philosopher from ancient Greece, who is sometimes referred to as ‘the laughing philosopher’.
He is largely associated with being the first to put together the theory of atomism. The word atom comes from the ancient Greek word ‘atomos’, tomos meaning “cuttable” and the prefix of a meaning “not”.
His theory was that everything is made of these uncuttable atoms and void (empty space).
Together with his teacher Leucippus, they proposed that the sensation of various things were down to the shapes of the atoms that they were made from. For example, they believed water atoms were smooth and slippery (as such couldn’t hold onto each other, and flowed), that salt atoms were sharp and pointed (as they tasted sour), and that iron atoms were rough and hooked shaped (allowing them to cling tightly together).
That’s my quick summary of what I learnt about Democritus!
Sources: Philosophize This! (http://www.philosophizethis.org) | Wikipedia (http://www.wikipedia.com) | Internet Encyclopedia of Philosophy (http://www.iep.utm.edu) | Britannica (http://www.britannica.com/biography)