When we expand a binomial, $\displaystyle (x+y)^n$ I understand how the coefficient of $\displaystyle x^r$ will be $\displaystyle ^nC_r$, but not why the terms group together as they do in the form $\displaystyle x^{n-r}y^r$.

Why is this the case and not $\displaystyle x^ry^r$ for example?