7
Let's add a motorcycle traveling at 80 km/h.
We may say that the bike always travels at a steady 80km/h. At a constant 80km/h. After 60 minutes the bike moved 80 km from to the house. But measured from the car, the bike does not travel that far. After 60 minutes the bike moved only 30 km from the car. Therefore for me in the car, 'relative to me', the bike's speed is 30 km/h. Summary: the speed of the motorcycle is not the same for everybody. So far the classic way of seeing our world of space and time. Now Einstein enters the scene. Note.  I didn't say: "For me the bike 'seems' to travel at 30 km/hour." For me the bike does travel at only 30km/h.  If the motorcycle travels at the same speed as my car, then the bike stands still next to me. I stick my hand out of the car window and I feel that the bike remains immobile next to me. For me the bike doesn't 'seem' nor 'appears' to be immobile. The bike is indeed immobile next to my car. The fact that my car and the bike both move at the same speed on the ground is of no importance and doesn't mean anything if we investigate the behaviour of the bike from the car's point of view, or the behaviour of the car from the bike's point of view.  Special Relativity stipulates that there's no preferred point of view (called a frame of reference) to judge a moving object. The motorcycle considers itself immobile and the car moves. The car considers itself immobile and the motorcycle moves! This is a very important fact based on what Galilei already highlighted nearly 400 years ago (his 'principle' of relativity). In the interest of brevity I can not elaborate on this here. Keep in mind that only the relative speed between the car and the motorcycle is important. 

=> Go to ....
 Introduction
 Relativity for Dummies
 4D Spacetime Block Universe
 Reciprocal time dilation
 Reciprocal length contraction
 Lengths do not 'appear' contracted!
 Measuring the contracted train
 Constant light speed
 Where is the bullet?
 Train experiment
 Slower and shorter  case study
 Minkowski vs Loedel
 Spacetime video
 Further reading