In ancient Greece, most people thought that matter was made up of combinations of four elements: earth, air, fire and water.
There were a small number of Greeks who had a different idea. They believed that if there was a piece of wood for example, it could be cut into smaller and smaller pieces until it ended up as a piece of wood that was so small it couldn't be cut anymore.
The ancient Greek philosopher Demokritos (460-370 BCE) thought that matter was made up of millions of tiny, uncuttable pieces of that same matter. In fact, the word atom comes from the word 'atomos', which means uncuttable.
After discovering the electron in 1897, J J Thomson, proposed that the atom looked like a plum pudding. To explain the two types of electric charge, he suggested that the atom consisted of positive 'dough' with a lot of negative electrons stuck in it. This was consistent with the evidence available at the time: