When physicists are looking for something, they put together an experiment and use what they know to find what they don't know. An experiment may then lead to more questions, which require more experiments to answer. The experiment may be simple, or it may require a huge detector and accelerator to understand how particles work.

First, we will see how an early physics experiment performed by Rutherford, Geiger, and Marsden demonstrated that atoms have a small charged nucleus.