Factorial - Walkthrough

Let me trace factorial($4$) step by step. Each call waits for the next to return. factorial($4$) calls factorial($3$), which calls factorial($2$), which calls factorial($1$), which calls factorial($0$). factorial($0$) hits the base case and returns $1$.

Now the chain unwinds: factorial($1$) returns $1 \times 1 = 1$, factorial($2$) returns $2 \times 1 = 2$, factorial($3$) returns $3 \times 2 = 6$, factorial($4$) returns $4 \times 6 = 24$. Notice how results multiply back up. The recursion "remembers" each waiting call. This is the call stack in action.