Another primo find via boing2 -- a clip from Cosmos of Carl Sagan explaining the 4th dimension and tesseracts. I'd partially remembered this and had been hoping to see it again. I've watched the series a couple of times in the last few years when it's been re-run on Discovery Science but I've always managed to miss that episode (#10: the Edge of Forever). So, yay, internets!
This is my all-time favorite bit from Cosmos.