Why do humans cry when they are sad?

The long-running series in which readers answer other readers’ questions on subjects ranging from trivial flights of fancy to profound scientific and philosophical concepts

I understand that tears flush away foreign objects from the eye. But what advantage does crying have when one is feeling sad (or happy)? Perhaps it is to signal an extreme of emotion, but then why would a solitary sad person cry when there was no one around? David Dobbs

Post your answers (and new questions) below or send them to nq@theguardian.com. A selection will be published on Sunday.

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