Factorial - Problem Statement

Now you will compute $n!$, the product of all positive integers up to $n$. For example, $5! = 5 \times 4 \times 3 \times 2 \times 1 = 120$. You could use a loop, but factorial is perfect for recursion because it has natural structure: $n! = n \times (n-1)!$.

Every factorial depends on a smaller factorial. Before reading on, think about this: when should you stop computing? And why is a stopping point necessary?