| I am not a good math student, and I know that those mathematicians among you will downvote this crazy little theory my friend and I came up with during an argument with out calculus teacher, but here it is nevertheless:
Zero is a number where the number line changes from negative to positive (or vice versa). What is, then, infinity? Could it be the point where the positive numbers change back to negative? This means that instead of a number line we have a number circle, where 0 is on one end and infinity (positive and negative infinity is the same point in this case, just as -0 and +0 is the same point) is on the other. All negative numbers are on one half of this circle, and all positive numbers are on another. When adding two numbers on a number line, you are simply adding the distances between the numbers and 0. This means that infinity plus infinity is 0 - because you make a complete loop around the circumference of the circle and end up with 0 displacement (where you started). Infinity - infinity is 0. Infinity + N (where n is any number) is undefined but negative (it's clearly not infinity because you can see that we moved along the circle and are no longer at infinity).
Applying this little idea to real life, it would mean that if you go over the edge of the universe, you just come back from the other side.
(note: I have been told that a similar concept can be found somewhere on the web. I didn't see that one so I don't know how similar mine is to their's)
(another note: if you add imaginary numbers, I suppose it would become a number sphere but that's a bit much) | 
