I think I understand how the classical thought experiment of the light clock (light bouncing between two mirrors work). The typical explanation that is given is "the stationary observer sees the light travel a longer distance (the diagonal), so since light can't go faster than the speed of light, it must be time that slows down". That's all right. But let's imagine that instaed of the light there is a ball bouncing up and down, at a non-light speed, in fact, quite slowly. But a constant speed. So now the moving observer carrying the clock sees the ball taking time x to do a vertical movement between the walls. The stationary observer sees the ball taking time x to do a slightly longer trip (the diagonal). Does this also mean that time slows down for the moving observer? Or that the ball moves faster for the stationary observer? How can the ball move at two different speeds at the same time?