Let’s consider Galileo’s description of relativity of motion using the example of a moving ship. The ship is at rest and made to accelerate extremely slowly for a few minutes, and then moves at a constant velocity. When the ship is moving at some constant velocity, the contents of the ship will also move with the ship and we will not be able to tell if the ship is moving or not. A box on the floor of the ship will obviously move with the ship and so can’t help us. The smoke emitted from a cigarette lying on a table in the ship will also follow the ship. This can be explained because as the particles of the smoke are emitted from the cigarette, these particles attain the velocity of the cigarette which is that of the ship. So the smoke continues to move forward with the ship. Same explanation for dropping water drops… A butterfly flying around in the ship has the ship’s interior as its local reference, and so follows the ship How about a mechanical “fly” hovering in midair inside the cabin? Will it also follow the ship? How to explain this phenomenon? Is it the air in the ship, which the mechanical fly is using to stay put, making the fly to move with the ship? What about a tiny rocket, burning rocket fuel, hovering midair in the ship and the cabin is a vacuum? Will it move with the ship? How? Would the tiny rocket be considered as another inertial reference frame which we are using to detect motion? The tiny rocket is a very simple one, burning the fuel just at the correct rate to stay put. There is no electronics or any such fancy stuff to control its location. Maybe I am missing something?