What has stuck with me since the first time I saw it was Carl Sagan's explanation for the speed of light and what would happen if you were to travel close to it. From "Cosmos": Part 1, 2. Very bizarre stuff that makes your brain itch.
Oh, man, I loved watching that show. They showed it on PBS every Saturday morning and it was my show. My high school had the textbook that went with that series. I was convinced I was going to be a physics major in college, get a phd and all that.