Looking back a long (long) way to my physics degree, special relativity was one of my favourite subjects. It's weird and wonderful enough to be amazing, but (unlike general relativity) the maths is relatively easy. Don't worry though, I'm not going to throw equations at you - I just wanted to share one of the remarkable paradoxes of relativity. I've seen paradoxes defined as contradictions that can't be true, but I think a much more appropriate definition for physics is situations that appear to involve a mind-boggling contradiction, that the physics tells us really is the way things are. And special relativity is full of them. This particular one below I hadn't seen before, and I picked up from Andrew Stearne's book The Wonderful World of Relativity . (This sounds like a children's book, but actually it's a relativity primer that is probably best appreciated by those about to start a physics course at university, as it's a bit too textbooky ...