I just got my first official introduction to relativity in my textbook (I'm equivalent to a high school senior student). It is part of a chapter on modern physics.It first mentions the two basic postulates and then briefly introduces what length contraction, time dilation, increase in mass, energy mass equivalence (all with an equation each) and speed of light as a limit are. I am a little confused with time dilation. According to the textbook, the clock at rest in a reference frame appears to tick slower to an observer moving with respect to that frame of reference.The time interval they measure is t' = t/√(1-v2/c2) , where t is the time interval as measured at rest in the frame of reference of the clock. 1-)To me it seems that this means that the moving observer will measure a longer time interval, meaning the time in the reference frame of the clock has slowed down from his point of view. But that's exactly opposite to what I've read about the decay of pions where time seems to slow down for the fast moving particle as viewed by an observer in the lab's frame of reference. 2-) Also in the topic just before relativity, the textbook mentions that motion and rest are relative. So to an observer at rest in the reference frame with the clock, the other observer (moving w.r.t the frame with the clock) seems to be moving. But then to the moving observer (in his own reference frame) this clock and its frame of reference seems to be moving. Shouldn't each observer see the other's clock ticking slower? What would that mean???