The span is basically the set of all linear combinations of the vectors w1, w2, ... , wn. So then, I can define some vector S and some vector T in terms of w's:
S = c1*w1 + c2*w2 + ... + cn*wn
T = k1*w1 + k2*w2 + ... + kn*wn
I think I get it now. I can see how S + T will be closed, and...