I once tried to build a differential as part of a school project. I understood the concept of it pretty well, but couldn’t find the parts I needed to make it small enough to use.
This is probably the best explanation I’ve found on the internet. The interesting stuff starts around 1:40.
I’m normally not very impressed by the BBC Technology blog (Rory Cellan-Jones obsessing over Twitter and iPhones), but I think today’s post highlighting the internet traffic management in Iran over the last few days is quite interesting.