Short answers, pulled from the story.
Sadness is an emotional pain associated with feelings of disadvantage, loss, despair, grief, helplessness, disappointment, and sorrow. A person experiencing sadness may become quiet or lethargic and withdraw from others, and crying can be an indication of it.
Paul Ekman described six basic emotions: sadness, happiness, anger, surprise, fear, and disgust. Sadness is one of them.
According to the American Journal of Psychiatry, sadness is associated with increases in bilateral activity near the middle and posterior temporal cortex, the lateral cerebellum, the cerebellar vermis, the midbrain, the putamen, and the caudate. Studies have also found increased activity in the prefrontal cortex, including Brodmann's area 9, the thalamus, and the bilateral anterior temporal structures.
Sadness is believed to serve two primary functions that help people cope with loss. One promotes cognitive changes that restructure beliefs and goals and reevaluate implications, and the other signals a need for assistance and elicits support from others.
Sadness is mostly communicated by lowering the mean and variability of the fundamental frequency, along with lower vocal intensity and decreases in f0 over time. DIPR scientist Swati Johar states that when someone is sad, slow, low pitched speech with weak high audio frequency energy is produced.
A sad facial expression with small pupils is judged to be more intensely sad as the pupil size decreases, and a viewer's own pupils mirror this by becoming smaller. The greater the degree to which a person's pupils mirror another's predicts a higher score on empathy.
In a study by Klaus Scherer, sadness in Western countries had 79% accuracy for facial recognition and 71% for vocal recognition. In Non-Western countries the results were 74% and 58%, respectively.