SRT Twin paradox
Minkowski diagram. You can select a blue observer and a green observer and decide whether they are stationaly or moving (move observers around by littles circles). To start with, blue is a stationary observer. AC is direct route from event A to event C ABC is indirect route from event A to event C via event B. Move event B and C around and see what happens. QuadranceBlueAB stands blue's x^2-t^2. Switch on hyperbole A to see de hyperbole with events equidistant from A. With slider "a" control the "radius" of the hyperbole. Also, for event B the equidistant hyperbole is available. Play around. Discover that the indirect route is shorter. The more one travels through space, the less one travels in time.
Switch on the perspectives of two observers (blue and green). The coordinates of the events A, B and C are different for green as for the stationary observer. However, both observers agree about the (relativistic) distances (proper time) between any two events. You can play with both observers. The (moving) observers will always agree about the proper time between events. They will almost never agree about their own time and distance coordinates (except through Lorentz transformations). Excercises: 1/ Can Blue and Green agree about a common origin? 2/ Can they agree about a unit length?