problem: If $x$ is an integer, what is the smallest value of the expression $x^2 - 6x +13$?
solution: We can write $x^2-6x+13 = x^2-6x+9+4 = (x-3)^2 + 4$. Therefore, since $(x-3)^2$ can never be negative, but we can make it zero when $x=3$, the smallest possible value of the expression $x^2-6x+13$ when $x$ is an integer is $\boxed{4}$.