I just finished listening to this week's Astronomy Cast. In it Dr. Pamela Gay explains how asking what the universe is expanding into is a nonsense question.

Now I have had some business rules over the years that were essentially this: "if it is the third Tuesday and there is a full moon in the Libra and you roll a 15 on a 20 sided die then ...".  Those seem like mild Excedrin days compared to trying to understand how the universe can be three dimensional and flat at the same time.  Then there is the whole planes on longitude lines and donuts explanation.  I'll let you know when my head stops throbbing.

Seriously though, check out the show.  It is a lot of fun if you are into astronomy.