Principle of relativity
Galileo's principle of relativity states "It is impossible by mechanical means to say whether we are moving or staying at rest". If two trains are moving at the same speed in the same direction, then a passenger in either train will not be able to notice that either train is moving. However, if the passenger takes a fixed frame of reference, a fixed point, like the earth, he will then be able to notice the motion of either train. Another thing, if one stands on the earth one will not be able to see that it is moving.