It is easy to visualise the shape of ordinary objects because you can look at them from the outside. You can pick up a cup, view it at different angles, feel its curves and surfaces, poke a finger through the handle. But we can't do this with the universe because we're stuck inside it - in fact, according to some definitions, it is all that exists. There are, however, still ways to feel its shape from the inside.
--- Simon Battersby, "Fold testament: The shape of the universe," New Scientist, 7 December 2006