I forget when I learned about special relativity, sometime in Junior or Senior High School, I think. I always had the most basic of understanding. At some point, I read Stephen Hawking’s A Brief History of Time and I understood a bit more.
For some reason, my web meanderings today took me to a Wikipedia article on the Twin Paradox. What always confused me was to think what images each would see if the twin on earth had a telescope trained on the spaceship and vice versa for the twin on the spaceship. Well this Minkowski diagram explains the whole thing. Wow.
Read the whole article if you were still unclear on the whole Twin Paradox thing.