text: A train is moving from station A to station B; the distance between them is 10km.\n\nWhen the distance is 2m from station A, a man at station A will say that the train is 2km from A; whereas the man at B will say that the train is at 8km.\n\nTherefore, relative to man at A, the distance is 2km and relative to the man at B the distance is 8km.\n\nThis is called relativity.\n\nIn Physics we measure the quantities like mass, length and time.\n\nThese quantities can be measured in a laboratory which is at rest.\n\nThe same quantities can be measured in a laboratory which is set up in a fast moving train.\n\nWill the observations will be the same or different?\n\nThe theory of relativity deals about such things.