Show the isomorphism between cellular and singular homology is natural in the following sense: A map f:X→Y that is cellular --- satisfying f(Xn)⊂Yn for all n --- induces a chain map f∗ between the cellular chain complexes of X and Y, and the map f∗:HnCW(X)→HnCW(Y) induced by this chain map corresponds to f∗:Hn(X)→Hn(Y) under the isomorphism HnCW≈Hn.