I'm to show that sin(x) is orthogonal to sin(x).

From class, we're told that f(x) and g(x) are orthogonal if

$\displaystyle \int _{-\pi }^{\pi }f (x) g(x)dx=0$

I am confused to begin with because when I graph sin(x)sin(x) I see that the area under the curve from -pi to pi is not zero. Can someone help me here please.