Crying emotional tears is uniquely human. We cry over almost anything and for almost any reason – from tears of sadness to tears of joy. Music can induce them, films, stories and television news too. We do not produce tears when we are first born – it takes a few months until we are able to. But once we can, we do it right up until our final days. So why do we cry? Mike Williams traces some of the competing theories of tears with the help of scientists, psychologists, and a historian. He also watches as an actress is made to cry by her acting coach.

(Image: A tear drops falls from a person's eye . BBC copyright)