Fix the flag on a thin pole and then tie it to a balloon. Can the flag float?
When the flag is fixed on the flagpole, it is stationary. When there is wind, the air is moving relative to the flag, which makes the flag fly. But the flag is tied to the balloon, because the balloon moves with the wind, and the wind and the flag also move at the same speed and direction, so the air is static relative to the flag at this time, and the fixed flag will not fly.