"Thinking about the immortality of the crab" (Spanish: Pensando en la inmortalidad del cangrejo) is a Spanish idiom about daydreaming. It is a humorous way to say that one was not sitting idly but engaged constructively in contemplation or letting one's mind wander.
The phrase is usually used to express that an individual was daydreaming, "When I have nothing to do I think about the immortality of the crab" (Cuando no tengo nada que hacer, pienso en la inmortalidad del cangrejo). It is also used to wake someone from a daydream; "are you thinking about the immortality of the crab?" (¿Estás pensando en la inmortalidad del cangrejo?)