Einstein's Special relativity: Continuing explain...

Today I want to continue explaining about Einstein Special relativity.
BUT! I forgot to say I'm not major in physic, so there may be mistake. Please tell me if you notice my mistake.



To know what is whatever you want, what will you do? It is effective to know or understand what will happen result from it. So I want to explain "light clock".
This is made from a light resource and a mirror these can move parallel with right angled direction of straight  between the mirror and the resource. In addition you already know the distance between the mirror and the light resource and the light velocity---this is constant. Then light from the light resource, the mirror reflects light, light comes back. You can calculate a time that is used to go and back between two. You can use this time as clock.
"Hmm, clock? This is a kitchen timer, isn't it?" 
Oh, it is likely and exactly. But do you know what property "clock" has to have? It is only Kitchen timer! If you want to know one second using this light clock, you should set a distance  299 792 458 m. The distance is distance light can reach in one second.
Anyway, we shall go to next step. The next experiment is to move equipments! Move equipments as velocity whatever you want to move these. Then imagine light from light resource, light go and come back..., but equipments are moving.  Look at from outside of equipments, light goes more distance than the experiment we had just before. 
Please DON'T be cheated by common sense!
Light velocity is constant, so light use more time to go the mirror. Why? Line light goes is oblique across the light resource and the mirror. If we've set the distance  299 792 458 m, more than one second is used to go one light velocity second. What is happen? Time lags!
You can't believe this fact, but many experiment shows time lag when you move or I move. "Move " is important factor. It is difficult to explain other experiments, but you will see those in    wikipedia: Special_relativity and so on. If you are not good at math, you can read Einstein's paper about Special relativity. It is very good way to know and think about Special relativity to read it.