You’re on a train moving 1,000mph. You’re sitting on top of the front car (don’t ever do that) and shoot a gun in the same direction in which the train is moving. Let’s say the gun also shoots bullets at 1,000mph.
Relative to the train (and you) the bullet will shoot ahead of the train at 1,000mph. However, to a viewer on the side of the tracks, since the bullet was moving at 1,000mph before it was even shot, it will be moving at 2,000mph according to their point of view.
Even cooler? Let’s say you’re on the back car of the train facing directly opposite of the direction it is moving (looking out the back window, basically). You shoot a bullet in that direction and what happens? It moves 1,000mph in the direction you shot it, relative to you. It behaves normally from your point of view. To an observer on the ground along the tracks? The bullet is traveling 1,000mph before being shot, and once shot, it travels 1,000mph in the opposite direction. So, while it moves 1,000mph relative to the person shooting the gun, to an observer on the ground, the bullet will appear to merely drop to the ground from the point at which it was fired.
Amazing. Magic does exist in our world. It’s called physics.