The world around us, everything we see, touch, smell and taste, is made of matter. Everything from the device you are reading this webpage on and the ground that you are standing on to your body and the air you breathe - all consist of matter. But what is "matter"? People have been puzzling over this question for generations. In the early 20th century, it was believed that the smallest unit of matter was the atom (from greek atomos, meaning indivisible). It was known that atoms of different types have different properties and cannot be transformed into one another. However, today we know what atoms are made up of and why the different atoms have different properties.