Mathematics pupils first learn about the irrational numbers $\pi$ and $e$ at the secondary school level. While the appearance of $\pi$ in simple geometrical formulas makes it easy for pupils to grasp its special importance, the significance of $e$ is less clear. The nature of $e$ can best be understood from a historical perspective, In the sixteenth century, work on various mathematical problems led, along different paths, to the discovery of~$e$. In this article, I will outline these paths, and propose that describing them to pupils is the best way to help them understand the uniqueness of~$e$.