Let $K$ be a simplicial complex and if $\sigma$ is a simplex of $K$.
How to prove that the following sets are simplicial complexes?
1) The boundary of $K$: $\partial(K)=\{\mbox{proper faces which belong to all the simplexes of K}\}$
2) The closure of $\sigma$: $\mathrm{Cl}(\sigma)=\{\mbox{faces of }\sigma\}$
Can you help me please? Thank you!!!