Scientific explanations often make use of things that cannot be seen or felt, such as protons, electrons, and quarks. Do these things really exist? If so, how do is it known that they exist? Imagine being enclosed in a completely dark room with no light at all and not being able to see a thing. Being chained to a chair somewhere in the room, and having a pile of rocks at disposal. While trying to decide whether or not this is part of a bad horror film, try decide to figure out the size of the room and its shape. The experiment described in this article gives a particular picture of what atoms look like.