We know that moving charges produce magnetic fields. Now, there are two observers in two different intertial frames: observer A is on the ground and observer B is on a train moving at some speed relative to observer A. Now suppose that there is stationary charge in the train according to observer B. The problem is that to observer A, the charge would be moving and therefore would be producing a magnetic field whereas to observer B the charge would be stationary and would produce no magnetic field! How does relativity resolves this apparent paradox??