  We prove a vertex-isoperimetric inequality for [n]^(r), the set of all
r-element subsets of {1,2,...,n}, where x,y \in [n]^(r) are adjacent if |x
\Delta y|=2. Namely, if \mathcal{A} \subset [n]^(r) with |\mathcal{A}|=\alpha
{n \choose r}, then the vertex-boundary b(\mathcal{A}) satisfies
|b(\mathcal{A})| \geq c\sqrt{\frac{n}{r(n-r)}} \alpha(1-\alpha) {n \choose r},
where c is a positive absolute constant. For \alpha bounded away from 0 and 1,
this is sharp up to a constant factor (independent of n and r).
