I'm trying to solve a problem which asks me to deduce that "the pullback of a pullback square is a pullback", using the result of http://ncatlab.org/nlab/show/pullback (under 'Pasting of pullbacks') that after concatenating 2 commutative squares into a larger rectangle, if the right-hand square is a pullback, the left-hand square is a pullback iff the larger rectangle is a pullback.
So, I gather I'm meant to be forming some sort of cube and using the result to show various combinations of faces make pullbacks, but I'm not sure what it actually means to say "the pullback of a pullback": I guess a pullback square is in a sense 2 commuting morphisms which you could take the pullback of, but when the problem says "the pullback of a pullback", clearly they don't just mean to pull back these 2 commuting morphisms because that's obviously a pullback. I think I'm meant to show that something is in fact that pullback of these 2 commuting morphisms, but what is it? Thanks for the help!