Concrete is a familiar material used widely in our daily lives, yet the reason it hardens is not widely understood. In this lecture, I will explain the basic mechanism by which concrete becomes solid. The key is a chemical reaction between cement and water, called hydration. This reaction produces new substances, including C-S-H, and as these hydration products grow and connect with one another, they form a skeleton that gives concrete its strength and solidity. Through this example, I hope to share the importance and fascination of thinking about why materials behave the way they do.