Relativity is quite counter-intuitive. Before relativity and quantum mechanics people thought of the physical world as behaving in ways that could be observed.
Einstien put together a theory (which explained some unexplained phenomena) that said all kinds of unintuitive things like:
As an object is accelerated it's mass increases
When an observer accelerates, the universe around him contracts in length in his direction of travel
When a clock is accelerated, it slows down (cause time slows down)
Even if a space ship is moving at 0.99 times the speed of light from the earth, to an observer in the space ship, light takes the same time to travel from the front of the ship to the back as from the back to the front.