The latest meme to hit the mathematics blog world is to explain why a negative number times a negative number is a positive number. All the posts below are worth reading. (EDIT: I have included extra posts made after this one.)
Crossed Streams: When a minus times a minus equals a plus
Walking Randomly: Why is a ‘minus times a minus equal to a plus’?
Math Less Traveled: Minus times minus is plus
Hans Gilde’s weblog: Why a negative times a negative is a positive
mathrecreation: beautiful negatives
code monk: Minus times a minus is a plus
Points of Intersection: A negative times a negative
I thought I’d make my own contribution. Click the image for a larger version.