Special Relativity. E.g. how could it be that observers A and B (moving relatively to each other) both think that time passes more slowly for the other one? How

Special Relativity. E.g. how could it be that observers A and B (moving relatively to each other) both think that time passes more slowly for the other one? How is that not a contradiction? Man, that took me a while. The solution: A and B have different notions of simultaneity and won't agree on which events occur at the same time. In particular, they won't agree on time measurements.