Induction

Suppose that you are trying to prove that for every positive integer $n$,

$$1^2 + 2^2 + ... + n^2 = {n(n+1)(2n+1) \over 6}$$For the induction step, you need to show that

$$1^2 + 2^2 + ... + k^2 + (k+1)^2 = ?$$