"What Shape is the Universe?"

Pick a friend and head down to the equator. The specific country doesn't matter; just go. Stand a few feet apart and face north (pro-tip: pack a compass). Start walking, one baby step at a time, never deviating from a northern course.

Even though the two of you start perfectly parallel, you'll eventually end up bumping into each other. The exact place where this happens has a name: the north pole.

Congratulations, you've just proved that the Earth is curved.

That is, in fact, the very definition of a curved surface: lines that start out parallel end up not being parallel. Take out a globe and look at all the lines of longitude (the up-and-down ones): they're parallel at the equator but all intersect at the poles. If the Earth were flat, those parallel lines would stay parallel forever.

Our universe, as far as we can tell, is flat. Parallel lines racing through space will always stay parallel. But like I said: that's as far as we can tell. Within our little observable bubble, everything is flat. But the actual universe is much larger than we can see, and it's shape is...well, we don't know. It's like trying to figure out the curvature of the Earth by just drawing lines in your back yard.

Here's something to bend your mind before I go. The universe can be geometrically flat, but topologically curved. Find a glass and draw parallel lines. Even though they wrap back around on themselves, they stay parallel, don't they? Cylinders are technically flat! So maybe if you rocket off into space in one direction far enough, you'll end up back where you started.