It is an important and popular fact that things are not always what they seem. For instance, on the planet Earth, man had always assumed that he was more intelligent than dolphins because he had achieved so much - the wheel, New York, wars and so on - whilst all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man - for precisely the same reasons.
Curiously enough, the dolphins had long known of the impending destruction of the planet Earth and had made many attempts to alert mankind of the danger; but most of their communications were misinterpreted as amusing attempts to punch footballs or whistle for tidbits, so they eventually gave up and left the Earth by their own means shortly before the Vogons arrived.
The last ever dolphin message was misinterpreted as a surprisingly sophisticated attempt to do a double-backwards somersault through a hoop whilst whistling the "Star Spangled Banner", but in fact the message was this: So long and thanks for all the fish!