If a hotel with an infinite number of rooms has an infinite number of guests, how could it free up space when new guests arrive? Hilbert's “Grand Hotel” paradox has fascinated mathematicians, physicists, philosophers, and theologians, as it encourages another way of thinking about the notion of infinity.

On the most fundamental level thinkable, what are things? Why are things? And why do things behave the way they do? Here is an introduction to particle physics, with all of the vocabulary you'll need.