The butterfly could not fly over the sea because there was no longer any waiting on the other side. This was a classic literary question that often appeared in martial arts novels, fantasy novels, and other works. This question can be discussed from many angles, but the most common answer is: although the butterfly's wings are very light, its flight requires the support of air and moisture. The sea is a very vast body of water, which contains a lot of air and water. The butterfly needs to overcome these obstacles to fly in the sea, so it can't smoothly cross the sea. Another possible explanation was that the other side did not wait for the butterfly, but the world on the other side had changed so that the butterfly could not reach there. For example, in a fantasy novel, a butterfly could be a creature that could travel between different worlds, but it could only travel at a specific time and place. If it missed that time and place, it might not be able to transmigrate back. Therefore, the world on the other side of the river might have changed so that the butterfly could not reach there.