Meaning of 'pullback of a pullback square' 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!

This CW answer intends to remove this question from the Unanswered queue.

The intended exercise is, as Zhen Lin points out:

Take 8 objects, and form a cube, with all arrows directed towards the back-right-bottom corner. Suppose the back, top, bottom, left, and right faces are pullback squares. Show the front is also a pullback square. (You actually only need some of the faces to be pullbacks.)

