The Fibonacci sequence (0, 1, 1, 2, 3, 5, 8, 13, ...) has a simple recursive definition: each number is the sum of the two before it.

def fib(n):
    if n < 2:
        return n
    return fib(n - 1) + fib(n - 2)

print([fib(i) for i in range(10)])

It reads exactly like the definition, but it is very slow: fib(30) makes over a million calls, because it works out the same smaller values again and again. fib(5) calls fib(3) twice and fib(2) three times, and it gets exponentially worse. Unit 36, on dynamic programming, fixes this by remembering answers. For now, notice that elegant isn't the same as efficient.