Well is when you do not have nothing to do, and think in all kind of superficial situation without importantce.
The Wikipedia give to us the next definition.
**Pensar en la immortalidad del cangrejo is a popular Spanish-speaking phrase or expression used to indicate that a person is distracted fantasizing about something. The phrase is often used humorously to indicate that a person is not paying attention to what they should be doing, but is in a state of contemplation or letting their mind wander. [ 1 ] It is also used to bring someone out of a daydream: "What are you doing, are you thinking about the immortality of the crab ? "
It's also a term used for when you don't want to do anything and go to bed to sleep, in which case you say goodbye by saying "goodbye, I'm going to think about the immortality of the crab."**
RE: Thoughts over coffee: Part One